Lesson 1: Light

3 Polarizer Experiment

Monday May 1, 2017

Shine a light through polarized sunglasses and the brightness decreases. If you hold two pairs of sunglasses one way, the light then is completely blocked! Not only that, but when you insert a third pair in between the two allows light to pass through again! Spooky!

Materials

Three pairs of polarized sunglasses (or three lenses from two old pairs)
Sunny window

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First, hold up one polarizer up between your eyes and the window. Notice how dim just one lens makes the incoming light from the window to your eye.
Rotate that polarizer 90^o. Does the light intensity (brightness) change? (It should not.)
Now stack a second polarizer in front of the first, so you’re looking through two polarizers to get to the window. (The image above shows a computer screen with two polarizers on the screen. The polarizers are in alignment with each other so that some light gets through.)
Rotate one of the polarizers 90^o so that the light is completely blocked. You should not be able to see the window at all through the polarizers. The image at the right shows the top polarizer rotated 90^o and blocks all the light from the computer screen.
Repeat steps 3 and 4 and play with it a bit, until you’re comfortable with the steps, then move onto the next step.

Imagine a picket fence—the kind with spaces between the wood. The polarizers are like picket fences in that they block out light that is in a different direction. When you rotate one of the polarizers so that it’s 90^o from the first one, it’s like rotating one picket fence 90o so that there are now very few gaps for light to get through.

Make sense so far? Now let’s add the last piece:

Insert a third polarizer in between the first two at a 45^o angle to each of them, like in the image at the right. What do you see happen?

What’s Going On?

The secret to making sense of this mystery is taking a look at one polarizer at a time.

Imagine having a polarizer that has it’s lines running vertically, like a picket fence. Any light that is also vertical will be able to pass through.

Sunlight is unpolarized, meaning that it’s in all directions. When it hits the first polarizer, only light that has components in the up-down vertical direction may pass through.

So if the incoming light is all completely vertical, then all the light will pass through and not lose any brightness at all.

If all the incoming light is horizontal, then none of it will pass through, since it’s all blocked. What if the light is at a 45^o angle?

Well, some of the light passes through and the rest does not, since light in this orientation has both vertical and horizontal components. Only the vertical component of the light is allowed to pass through the first filter, which in our example is about 71% of the light may pass through.

When that light hits the second filter, which is at a 45^o angle from the first polarizer, again some of it is allowed to pass through and the rest is blocked.

The same is true when the light hits the third polarizer. Some passes through and the rest is blocked. When you total it up, about 25% of the original incoming light passes through all three filters.

When one polarizer is at a 90^o angle from the second, then all the light is blocked, because none of the light coming out of the first polarizer has any components that are aligned with the second polarizer.

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Science Teleclass: Light & Lasers & Holograms

Sunday April 30, 2017

This is a recording of a recent live teleclass I did with thousands of kids from all over the world. I’ve included it here so you can participate and learn, too!

This class is all about Light Waves, Lasers and Holograms! This is a newly updated version of the older Light Waves and Lasers teleclass here.

We’re going to learn about the wild world of light that has baffled scientists for over a century. You’ll be twisting and bending light as we learn about refraction, reflection, absorption, and transmission using lenses, lasers, mirrors, and optical filters with everyday stuff like gummy bears, paperclips, pencils and water!

We’re going to learn how to build a projection hologram out of piece of old plastic, make a laser microscope so you can see tiny little microscopic creatures, bend laser light to follow any path you want without using mirrors, and finally understand how glow in the dark toys really work on the subatomic level. Are you ready?

Materials:

Pencil
Paper
Clothespin
Paperclip
Rubber band
Gummy bears
Red laser
Flashlight
Old CD
Scissors
Pliers
Glass of water
Clear Plastic Film

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Click here to download the worksheet for this class!

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Refractive Index

Thursday January 15, 2015

The refractive index provides a measure of the relative speed of light in that particular medium which allows us to figure out speeds in other mediums as well as predict which way light will bend.

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Click here to go to next lesson on Disappearing Glass

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Light Speed

Monday October 27, 2014

Particles that move close to the speed of light have a different equation for momentum in order for momentum to be conserved using Einstein’s relativistic equations. The speeds of large objects like baseballs, bullets, and satellites are so much less than the speed of light so we can use Newton’s equations for it. If you’re studying electrons and other subatomic particles, you must use equations from special relativity.

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Click here to go to next lesson on Center of Mass.

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Science Teleclass: Light & Lasers

Monday October 6, 2014

This is a recording of a recent live teleclass I did with thousands of kids from all over the world. I've included it here so you can participate and learn, too!

This class is all about Light Waves! Energy can take one of two forms: matter and light (called electromagnetic radiation). Light is energy in the form of either a particle or a wave that can travel through space and some kinds of matter, like glass.

We're going to investigate the wild world of the photon that has baffled scientists for over a century. We'll also do experiments in shattering laser beams, bending and twisting light, and also split light waves into rainbow shadows. Materials:

laser pointer
flashlight
paper clip
gummy bear (green and red)
old CD
paper clip
rubber band
pond water (just a little bit)

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Key Concepts

Imagine tossing a rock into a still pond and watching the circles of ripples form and spread out into rings. Now look at the ripples in the water - notice how they spread out. What makes the ripples move outward is energy , and there are different kinds of energy, such as electrical (like the stuff from your wall socket), mechanical (a bicycle), chemical (a campfire) and others.

The ripples are like light. Notice the waves are not really moving the water from one side of the pond to the other, but rather move energy across the surface of the water. To put it another way, energy travels across the pond in a wave. Light works the same way – light travels as energy waves. Only light doesn't need water to travel through the way the water waves do - it can travel through a vacuum (like outer space).

Light can change speed the same way sound vibrations change speed. (Think of how your voice changes when you inhale helium and then try to talk.) The fastest light can go is 186,282 miles per second – that's fast enough to circle the Earth seven times every second, but that's also inside a vacuum. You can get light going slower by aiming it through different gases. In our own atmosphere, light travels slower than it does in space.

Your eyeballs are photon detectors. These photons move at the speed of light and can have all different wavelengths, which correspond to the colors we see. Red light has a longer wavelength (lower energy and lower frequency) that blue light.

What's Going On?

When a beam of light hits a different substance (like a window pane or a lens), the speed that the light travels at changes. (Sound waves do this, too!) In some cases, this change turns into a change in the direction of the beam.

For example, if you stick a pencil is a glass of water and look through the side of the glass, you'll notice that the pencil appears shifted. The speed of light is slower in the water (140,000 miles per second) than in the air (186,000 miles per second), called optical density, and the result is bent light beams and broken pencils.

You'll notice that the pencil doesn't always appear broken. Depending on where your eyeballs are, you can see an intact or broken pencil. When light enters a new substance (like going from air to water) perpendicular to the surface (looking straight on), refractions do not occur.

However, if you look at the glass at an angle, then depending on your sight angle, you'll see a different amount of shift in the pencil. Where do you need to look to see the greatest shift in the two halves of the pencil?

Depending on if the light is going from a lighter to an optically denser material (or vice versa), it will bend different amounts. Glass is optically denser than water, which is denser than air.

Not only can you change the shape of objects by bending light (broken pencil or whole?), but you can also change the size. Magnifying lenses, telescopes, and microscopes use this idea to make objects appear different sizes.

Questions to Ask

Can light change speeds?
Can you see ALL light with your eyes?
Give three examples of a light source.
Why does the pencil appear bent? Is it always bent? Does the temperature of the water affect how bent the pencil looks? What if you put two pencils in there?
What if you use oil instead of water for bending a pencil?
How does a microscope work?
What's the difference between a microscope and a telescope?

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Tech Light Lab Experiments

Thursday March 20, 2014

This set of experiments will show you the properties of light, including optics, diffraction, transmission, reflection, wavelength, intensity, and so much more. You’ll discover how light travels in a straight line, how light can turn a corner, split into several beams, and why objects can appear dark even when light is shining right on them.

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Materials: You can order all these parts in one kit called the Tech Light Lab!

three flashlights
fingernail polish (red, blue, and green)
clear tape
small mirror
paperclip
old CD or diffraction grating
clear pieces to shine your light through
protractor
pencil
ruler
index cards (3)
paper
three objects: one red, one blue, and one green
aluminum foil
tack
water glass
binder clip (optional)

This is a longer video that has several experiments on it. I left them all together in one long video, as the experiments build on each other, and this set is best done all together. You should be able to complete all of the experiments in about 35-45 minutes. Here are the experiments in the video:

Diffraction Gratings
Does Light Travel in a Straight Line?
Exploring Shadows
Reflecting Light
Bouncing Light
Adding Light
Bending Light
Refraction

Download your student worksheet here!

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Sky in a Jar

Sunday September 1, 2013

Have you ever wondered why the sky is blue? Or why the sunset is red? Or what color our sunset would be if we had a blue giant instead of a white star? This lab will answer those questions by showing how light is scattered by the atmosphere.

Particles in the atmosphere determine the color of the planet and the colors we see on its surface. The color of the star also affects the color of the sunset and of the planet.

Materials

Glass jar
Flashlight
Fingernail polish (red, yellow, green, blue)
Clear tape
Water
Dark room
Few drops of milk

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Download Student Worksheet & Exercises

Make your room as dark as possible for this experiment to work.
Make sure your label is removed from the glass jar or you won’t be able to see what’s going on.
Fill the clear glass jar with water.
Add a teaspoon or two of milk (or cornstarch) and swirl.
Shine the flashlight down from the top and look from the side – the water should have a bluish hue. The small milk droplets scatter the light the same way our atmosphere’s dust particles scatter sunlight.
Try shining the light up from the base – where do you need to look in order to see a faint red/pink tint? If not, it’s because you are looking for hues that match our real atmosphere, and the jar just isn’t that big, nor is your flashlight strong enough! Instead, look for a very slight color shift. If you do this experiment after being in the dark for about 10 minutes (letting your eyes adjust to the lack of light), it is easier to see the subtle color changes. Just be careful that you don’t let the brilliant flashlight ruin your newly acquired night-vision, or you’ll have to start the 10 minutes all over again.
If you are still having trouble seeing the color changes, shine your light through the jar and onto an index card on the other side. You should see slight color changes on the white card.
Cover the flashlight lens with clear tape.
Paint on the tape (not the lens) the fingernail polish you need to complete the table.
Repeat steps 7-9 and record your data.

What’s Going On?

Why is the sunset red? The colors you see in the sky depends on how light bounces around. The red/orange colors of sunset and sunrise happen because of the low angle the Sun makes with the atmosphere, skipping the light off dust and dirt (not to mention solid aerosols, soot, and smog). Sunsets are usually more spectacular than sunrises, as more “stuff” floats around at the end of the day (there are less particles present in the mornings). Sometimes just after sunset, a green flash can be seen ejecting from the setting Sun.

The Earth appears blue to the astronauts in space because the shorter, faster wavelengths are reflected off the upper atmosphere. The sunsets appear red because the slower, longer wavelengths bounce off the clouds.Sunsets on other planets are different because they are farther (or closer) to the Sun, and also because they have a different atmosphere than planet Earth. The image shown here is a sunset on Mars. Uranus and Neptune appear blue because the methane in the upper atmosphere reflects the Sun’s light and the methane absorbs the red light, allowing blue to bounce back out.

Exercises

What colors does the sunset go through?
Does the color of the light source matter?

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Optics, Fire, and Eyes

Sunday May 29, 2011

If you’ve never done this experiment, you have to give it a try! This activity will show you the REAL reason that you should never look at the sun through anything that has lenses in it.

Because this activity involves fire, make sure you do this on a flame-proof surface and not your dining room table! Good choices are your driveway, cement parking lot, the concrete sidewalk, or a large piece of ceramic tile. Don’t do this experiment in your hand, or you’re in for a hot, nasty surprise.

As with all experiments involving fire, flames, and so forth, do this with adult help (you’ll probably find they want to do this with you!) and keep your fire extinguisher handy.

Materials:

sunlight
dead leaf
magnifying glass
fire extinguisher
adult help

Here’s what you need to do:

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Magnifying lenses, telescopes, and microscopes use this idea to make objects appear different sizes by bending the light. When light passes through a different medium (from air to glass, water, a lens…) it changes speed and usually the angle it’s traveling at. A prism splits incoming light into a rainbow because the light bends as it moves through the prism. A pair of eyeglasses will bend the light to magnify the image.

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‘Light’ Exercises

Wednesday February 3, 2010

Let’s see how much you’ve picked up with these experiments and the reading – answer as best as you can. (No peeking at the answers until you’re done!) Just relax and see what jumps to mind when you read the question. You can also print these out and jot down your answers in your science notebook.

Click here for a printer-friendly version of the Unit 9: Light & Lasers Exercises.

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1. Can light change speeds? How about sound waves?

2. Can you see ALL electromagnetic waves with your eyes?

3. Which has a longer wavelength, red or blue light? Which has more energy?

4. Give three examples of a light source.

5. Are radio waves the same thing as sound waves?

6. How does a microwave cook your food?

7. How is a snake like a TV remote?

8. Does UV light have more or less energy than visible light we can see with our eyes?

9. Is light a particle or a wave?

10. What was so cool about Einstein’s red light/ blue light experiment?

11. How do you make yellow light? Yellow paint?

12. What does a prism do?

13. How far do you need to rotate the sunglasses to block most (if not all) light?

14. Why does the pencil appear bent? Is it always bent?

15. How can you make a glass container disappear?

16.How does a microscope work?

Need answers?

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Answers to ‘Light’ Exercises

Wednesday February 3, 2010

Let’s see how you did! If you didn’t get a few of these, don’t let it stress you out – it just means you need to play with more experiments in this area. We’re all works in progress, and we have our entire lifetime to puzzle together the mysteries of the universe!

Here’s printer-friendly versions of the exercises and answers for you to print out: Simply click here for printable questions and answers.

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1. Light can change speed the same way sound vibrations change speed. (Think of how your voice changes when you inhale helium and then try to talk.) The “speed limit” of light is 186,282 miles per second – that’s fast enough to circle the Earth seven times every second, but that’s also inside a vacuum. You can get light going slower by aiming it through different gases. In our own atmosphere, light travels slower than it does in space.

2. No. Human eyes can only detect a small portion of all light (in the visible range).

3. Red light has a LONGER wavelength and LESS energy than blue light.

4. Campfire, the sun, and a neon OPEN sign.

5. No. Radio waves are LIGHT waves that are very low energy and have a loooooong wavelength.

6. By aiming light beams at your food which are specially tuned to excite the water molecule. Since all foods have water, this works to heat up your food. Excited molecules are ones that jiggle and zip around fast, which is also called heat

7. Both use IR (infrared) light. The snake is a detector and the TV remote is an emitter.

8. Longwave UV are black lights you can get around Halloween that make things glow and fluoresce, and these types of lights are not damaging to living tissue even though they have more energy than visible light. Short wave UV (which have shorter wavelengths and more energy), however, are damaging and can burn your skin.

9. Both, and you really can’t separate the two.

10. When you aim a blue light on a metal plate, electrons shoot off the surface. Red light doesn’t cause electrons to eject, however, no matter how bright you make the red light. It’s the wavelength, not the intensity that matters with the photoelectric effect.

11. Mix together green and red light to get yellow light. Yellow paint is a fundamental color that can’t be made from any others – you have to start with yellow.

12. A prism un-mixes the light beams into its separate colors.

13. The sunglasses need to be 90 degrees from each other.

14. The pencil appears bent (or broken) because the water and the glass change the speed of light. Depending on where your line of sight is, you can make the pencil appear broken or whole.

15. Besides hiding it in a closet, you can also place a Pyrex glass container inside a glass container filled with mineral oil, vegetable oil, or light Karo syrup. The index of refraction is the same for both, so our eyes are unable to see the difference between the two.

16. A microscope uses lenses that bend the light to make things appear larger. Using two convex lens magnifiers, you can find the tiny owl in the upper corner of the dollar bill that’s normally hidden to the naked eye.

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The Dark and Light of Polarization

Wednesday February 3, 2010

Polarization has to do with the direction of the light. Think of a white picket fence – the kind that has space between each board. The light can pass through the gaps int the fence but are blocked by the boards. That’s exactly what a polarizer does.

When you have two polarizers, you can rotate one of the ‘fences’ a quarter turn so that virtually no light can get through – only little bits here and there where the gaps line up. Most of the way is blocked, though, which is what happens when you rotate the two pairs of sunglasses. Your sunglasses are polarizing filters, meaning that they only let light of a certain direction in. The view through the sunglasses is a bit dimmer, as less photons reach your eyeball.

Polarizing sunglasses also reduce darken the sky, which gives you more contrast between light and dark, sharpening the images. Photographers use polarizing filters to cut out glaring reflections.

Materials:

two pairs of polarized sunglasses
tape (the 3/4″ glossy clear kind works best – watch second video below)
window

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Here’s what you do: Stack two pairs of sunglasses on top of each other and look through both sets of lenses… now rotate one pair a quarter turn (90^o). The lenses should block the light completely at 90^o and allow light to pass-through when aligned at 0^o. These lenses allow some light to pass through but not all. When you rotate the lenses to 90^o, you block out all visible light.

You use the “filter” principle in the kitchen. When you cook pasta, you use a filter (a strainer) to get the pasta out of the water. That’s what the sunglasses are doing – they are filtering out certain types of light. Rotating the lenses 90^o to block out all light is like trying to strain your pasta with a mixing bowl. You don’t allow anything to pass through.

Astronomers use polarizing filters to look at the moon. Ever notice how bright the moon is during a full moon, and how dim it is near new moon? Using a rotating polarizing filter, astronomer can adjust the amount of light that enters into their eye.

Download Student Worksheet & Exercises

Advanced students: Download your Polarization lab here.

Exercises

Why do you need two polarizers to block the light completely?
How can you tell if your sunglasses are polarized if you only have one pair?

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How to “See” Infra-Red Light

Wednesday February 3, 2010

Crazy Remote

Want to have some quick science fun with your TV remote? Then try this experiment next time you flip on the tube:

Materials:

metal frying pan or cookie sheet
TV remote control
plastic sheet

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Making IR Visible to the Human Eye

Infra-red light is in the part of the electromagnetic spectrum that isn’t usually visible to human eyes, but using this nifty trick, you will easily be able to see the IR signal from your TV remote, remote-controller for an RC car, and more!

TV remote control
camera (video or still camera)

Download Student Worksheet & Exercises

Exercises

Look over your data table. What kinds of objects (plastic, metal, natural, etc.) allow infrared light to pass through them?
Why does the camera work in making the infrared light visible?

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Light Tricks

Wednesday February 3, 2010

When light rays strikes a surface, part of the beam passes through the surface and the rest reflects back, like a ball bouncing on the ground. Where it bounces depends on how you throw the ball.

Have you ever looked into a pool of clear, still water and seen your own face? The surface of the water acts like a mirror and you can see your reflection. (In fact, before mirrors were invented, this was the only way people had to look at themselves.) If you were swimming below the surface, you’d still see your own face – the mirror effect works both ways.

Have you ever broken a pencil by sticking it into a glass of water? The pencil isn’t really broken, but it sure looks like it! What’s going on?

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When a beam of light hits a different substance (like the water), the wavelength changes because the speed of the light changes. If you’re thinking that the speed of light is always constant, you’re right… in a vacuum like outer space between two reference frames.

But here on Earth, we can change the speed of light just by shining a light beam through different materials, like water, ice, blue sunglasses, smoke, fog, even our own atmosphere. How much the light speed slows down depends on what the material is made of. Mineral oil and window glass will slow light down more than water, but not as much as diamonds do.

How broken the pencil appears also depends on where you look. In some cases, you’ll see a perfectly intact pencil. Other times, you’ll guess neither piece is touching. This is why not everyone can see a rainbow after it rains. The sun must be at a low angle in the sky, and also behind you for a rainbow to appear. Most times, you aren’t at the right spot to see the entire arc touch the ground at both ends, either.

Lenses work to bend light the way you want them to. The simplest lenses are actually prisms. Prisms unmix light into its different wavelengths. When light hits the prism, most of it passes through (a bit does reflect back) and changes speed. Since the sunlight is made up of many different wavelengths (colors), each color gets bent by different amounts, and you see a rainbow out the other side.

Double Your Money

Here are a few neat activities that experiment with bending light, doubling your money, and breaking objects. Here’s what you do:

Materials:

glass jar (or water glass)
penny
eyeballs

Download Student Worksheet & Exercises

Here’s what you do:

1. Toss one coin into a water glass (pickle jars work great) and fill with an inch of water. Hold the glass up and find where you need to look to see TWO coins. Are the coins both the same size? Which one is the original coin? (Answer at the bottom of this page.)

2. Look through the top of the glass – how many coins are there now? What about when you look from the side?

3. Toss in a second coin – now how many are there?

4. Remove the coins turn out the lights. Shine a flashlight beam through the glass onto a nearby wall. (Hint – if this doesn’t work, try using a square clear container.) Stick a piece of paper on the wall where your light beam is and outline the beam with a pencil.

5. Shine the light at an angle up through the water so that it bounces off the surface of the water from underneath. Trace your new outline and compare… are they both the same shape?

6. Add a teaspoon of milk and stir gently. (No milk? Try sprinkling in a bit of white flour.) Now shine your flashlight through the container as you did in steps 4 and 5 and notice how the beam looks.

7. Use a round container instead of square… what’s the difference?

Answers:
1. The smaller coin is the reflection.
2. One coin when glanced from above, two from the side.
3. Four.
4. Beam is a circle.
5. Beam is an oval.
6. I can see the beam through the water!!
7. The round container distorts the beam, and the square container keeps the light beam straight. Both are fun!

The coin water trick is a neat way for kids to see how refraction works. In optics, refraction happens when light waves travel from one medium with a certain refractive index (air, for example) to another medium which has a different refractive index (like water). At the boundary between the two (where air meets water), the wave changes direction.

The wavelength increases or decreases but the frequency remains constant. When you sine light through a prism, the wavelength changes and you see a rainbow as the prism un-mixes white light into its different colors.The light wave changed direction when it traveled from air to glass, and then back to air again as it leaves the backside of the prism.

Did you try the pencil experiment? Did you notice how if you look at the pencil (placed at a slant) partially in the water, it appears to bend at the water’s surface? The light waves bend as they travel from water to air. To further complicate things, the way the eye received information about the position of the pencil actually makes the pencil to appear higher and the water shallower than they really are! Can you imagine how important this is for trying to spear a fish? The fish might appear to be in a different place, so you need to account for this when you take aim!

Click here for the Disappearing Beaker experiment!

Exercises

When one coin is in the water, you can actually see two: Are the coins both the same size? Which one is the original coin?
In step 2 of the experiment: How many coins are there when viewed from the top of the glass? What about when you look from the side?
What happened when you tossed in a second coin?
How did your outlines compare?

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Disappearing Beaker

Wednesday February 3, 2010

We’re going to bend light to make objects disappear. You’ll need two glass containers (one that fits inside the other), and the smaller one MUST be Pyrex. It’s okay if your Pyrex glass has markings on the side. Use cooking oil such as canola oil, olive oil, or others to see which makes yours truly disappear. You can also try mineral oil or Karo syrup, although these tend to be more sensitive to temperature and aren’t as evenly matched with the Pyrex as the first choices mentioned above.

Here’s what you need:

two glass containers, one of which MUST be Pyrex glass
vegetable oil (cheap canola brand is what we used in the video)
sink

Published value for light speed is 299,792,458 m/s = 186,282 miles/second = 670,616,629 mph
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When a beam of light hits a different substance (like glass), the speed of light changes. The color of the light (called the wavelength) can also change. In some cases, the change of wavelength turns into a change in the direction of the beam.

For example, if you stick a pencil is a glass of water and look through the side of the glass, you’ll notice that the pencil appears shifted. The speed of light is slower in the water (140,000 miles per second) than in the air (186,000 miles per second), called optical density, and the result is bent light beams and broken pencils.

You’ll notice that the pencil doesn’t always appear broken. Depending on where your eyeballs are, you can see an intact or broken pencil.

This is a very fine point about refraction: when light enters a new substance (like going from air to water) perpendicular to the surface (looking straight on), refractions do not occur.

However, if you look at the glass at an angle, then depending on your sight angle, you’ll see a different amount of shift in the pencil. Where do you need to look to see the greatest shift in the two halves of the pencil? (Hint: move the pencil back and forth slowly.)

Depending on if the light is going from a lighter to an optically denser material (or vice versa), it will bend different amounts. Glass is optically denser than water, which is denser than air.

Here’s a chart:

Vacuum 1.0000
Air 1.0003
Ice 1.3100
Water 1.3333
Pyrex 1.4740
Cooking Oil 1.4740
Diamond 2.4170

This means if you place a Pyrex container inside a beaker of vegetable oil, it will disappear. This also works for some mineral oils and Karo syrup. Note however that the optical densities of liquids vary with temperature and concentration, and manufacturers are not perfectly consistent when they whip up a batch of this stuff, so some adjustments are needed.

Not only can you change the shape of objects by bending light (broken or whole), but you can also change the size. Magnifying lenses, telescopes, and microscopes use this idea to make objects appear different sizes.

Questions to Ask

Does the temperature of the oil matter?
What other kinds of oil work? Blends of oils?
Does it work with mineral oil or Karo syrup?
Is there a viewing angle that makes the inside container visible?
Which type of lighting makes the container more invisible?
Can we see light waves?

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Microscopes and Telescopes

Wednesday February 3, 2010

Hans Lippershey was the first to peek through his invention of the refractor telescope in 1608, followed closely by Galileo (although Galileo used his telescope for astronomy and Lippershey’s was used for military purposes). Their telescopes used both convex and concave lenses.

A few years later, Kepler swung into the field and added his own ideas: he used two convex lenses (just like the ones in a hand-held magnifier), and his design the one we still use today. We're going to make a simple microscope and telescope using two lenses, the same way Kepler did. Only our lenses today are much better quality than the ones he had back then!

You can tell a convex from a concave lens by running your fingers gently over the surface – do you feel a “bump” in the middle of your hand magnifying lens? You can also gently lay the edge of a business card (which is very straight and softer than a ruler) on the lens to see how it doesn't lay flat against the lens.

Your magnifier has a convex lens – meaning the glass (or plastic) is thicker in the center than around the edges. The image here shows how a convex lens can turn light to a new direction using refraction. You can read more about refraction here.

A microscope is very similar to the refractor telescope with one simple difference – where you place the focus point. Instead of bombarding you with words, let’s make a microscope right now so you can see for yourself how it all works together. Are you ready?

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How to Make a Microscope

Materials:

2 hand held magnifiers
dollar bill
penny - Note: The penny used in this video shows the Lincoln Memorial, which was shown on pennies minted between 1959 - 2008.

Here's what you do: Hold one magnifying glass in each hand. Focus one lens on a printed letter or small object. Add the second lens above the first, so you can see through both. Move the lens toward and away from you until you bring the letter into clear focus again. You just made a microscope! The lens closest to your eye is the EYEpiece. The lens closest to the object is the OBJECTive. The image here is of the objective part of a compound microscope. The different silver tubes have different sizes of lenses, each with a different magnification, so the same scope can go from 40X to 1,000X with the flip of a lens.

How do I determine magnification power for my microscope? Simply multiply the powers of your optics together to get the power of magnification. If you’re using one lens at 10X and the other at 4X, then the combined effect is 40X. You’ll usually find the power rating stamped in tiny writing along the magnifier.

So now you've made a microscope. How about a telescope? Is it really a lot different?

The answer is no. Simply hold your two lenses as you would for a microscope, but focus on a far-away object like a tree. You just made a simple telescope… but the image is upside-down!

microscope1 We don’t fully understand why, but every time we teach this class, kids inevitably start catching things on fire. We think it’s because they want to see if they really can do it – and sure enough, they find out that they can! Just do it in a safe spot (like a leaf on concrete) if that’s something you want to do. Click here for a detailed instructional video on how to do this safely.

How do I connect the flaming shrubbery back to the main optics lesson? Ask your child why the leaf catches on fire… and when the shrug, you can lead them around to a discussion about focus points of a lens. It’s hard for kids to visualize the light lines through a lens, so you can shine a strong light through a fine-tooth comb as shown in the image above. Use clear gelatin (or Jell-O) shapes as your “lenses” and shine your rays of light through it. If your room is dark enough, you’ll get the image shown above.

The point where all the lines intersect is where things catch fire, as the energy is most concentrated at this point. Note how the lines flip after the focus point – this is why the telescope images are inverted. The microscope image is not flipped because you’ve placed the image (and/or your eye) before the focus point. Play around with it and find out where the focus point is. Slide your lenses along a yardstick to easily measure distances.

How to Make a Telescope

Materials:

2 hand held magnifiers
window

Want to experiment further? Then click for the Optical Bench experiment and also sneak a peek at the Advanced Telescope Building experiment where you will learn about lenses, refractor, and newtonian telescopes.

Ready to buy your own professional-quality instrument that will last you all the way through college? Click here for our recommendations on microscopes, telescopes, and binoculars.
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Spectrometer

Wednesday February 3, 2010

Spectrometers are used in chemistry and astronomy to measure light. In astronomy, we can find out about distant stars without ever traveling to them, because we can split the incoming light from the stars into their colors (or energies) and “read” what they are made up of (what gases they are burning) and thus determine their what they are made of. In this experiment, you’ll make a simple cardboard spectrometer that will be able to detect all kinds of interesting things!

SPECIAL NOTE: This instrument is NOT for looking at the sun. Do NOT look directly at the sun. But you can point the tube at a sheet of paper that has the sun’s reflected light on it.

Usually you need a specialized piece of material called a diffraction grating to make this instrument work, but instead of buying a fancy one, why not use one from around your house? Diffraction gratings are found in insect (including butterfly) wings, bird feathers, and plant leaves. While I don’t recommend using living things for this experiment, I do suggest using an old CD.

CDs are like a mirror with circular tracks that are very close together. The light is spread into a spectrum when it hits the tracks, and each color bends a little more than the last. To see the rainbow spectrum, you’ve got to adjust the CD and the position of your eye so the angles line up correctly (actually, the angles are perpendicular).

You’re looking for a spectrum (the rainbow image at left) – this is what you’ll see right on the CD itself. Depending on what you look at (neon signs, chandeliers, incandescent bulbs, fluorescent bulbs, Christmas lights…), you’ll see different colors of the rainbow. For more about how diffraction gratings work, click here.

Materials:

old CD
razor
index card
cardboard tube

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Download Student Worksheet & Exercises

Find an old CD and a cardboard tube at least 10 inches long. Cut a clean slit less than 1 mm wide in an index card or spare piece of cardboard and tape it to one end of the tube. Align your tube with the slit horizontally, and on the top of the tube at the far end cut a viewing slot about one inch long and ½” inch wide. Cut a second slot into the tube at a 45 degree angle from the vertical away from the viewing slot. Insert the CD into this slot so that it reflects light coming through the slit into your eye (viewing slot).

Aim the 1 mm slit at a light source such as a fluorescent light, neon sign, sunset, light bulb, computer screen, television, night light, candle, fireplace… any light source you can find. Look through the open hole at the light reflected off the compact disk (look for a rainbow in most cases) inside the cardboard tube.

Troubleshooting: This is a quick and easy way to bypass the need for an expensive diffraction grating. Use your spectrometer to look at computer screens, laptops, night lights, neon lights, candles, campfires, fluorescent lights, incandescent lights, LEDs, stoplights, street lights, and any other light sources you can find, even the moon through a telescope.

To make a CALIBRATED Spectrometer, go here.

Exercises

Name three more light sources that you think might work with your spectroscope.
Why is there a slit at the end of the tube instead of leaving it open?

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Pinhole Camera

Wednesday February 3, 2010

This is the simplest form of camera – no film, no batteries, and no moving parts that can break. The biggest problem with this camera is that the inlet hole is so tiny that it lets in such a small amount of light and makes a faint image. If you make the hole larger, you get a brighter image, but it’s much less focused. The more light rays coming through, the more they spread out the image out more and create a fuzzier picture. You’ll need to play with the size of the hole to get the best image.

While you can go crazy and take actual photos with this camera by sticking on a piece of undeveloped black and white film (use a moderately fast ASA rating), I recommend using tracing paper and a set of eyeballs to view your images. Here’s what you need to do:

Materials:

box
tracing paper
razor or scissors
tape
tack

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Download Student Worksheet & Exercises

Here’s the quick set of instructions:

1. Use a cardboard box that is light-proof (no leaks of light anywhere).
2. Seal light leaks with tape if you have to. Cut off one side of the box (Note – there’s no need to do this if you’re using a shoebox).
3. Tape a piece of tracing paper over the cutout side, keeping it taut and smooth.
4. Make a pinhole in the box side opposite of the tracing paper.
5. Point the pinhole at a window and move toward or away from the window until you see its image in clear focus on the tracing paper.

OPTIONAL: You can hold up a magnifying glass in front of the pinhole to sharpen the image.

Exercises

How do the images appear when they’re projected onto the paper inside your camera?
Why do you think it’s important to make the box as light-proof as possible?
Is there a part of your body that works similarly to the pinhole?
Sketch a picture of something you saw through your pinhole camera.

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Mixing Cold Light

Wednesday February 3, 2010

Here’s a trick question – can you make the color “yellow” with only red, green, and blue as your color palette? If you’re a scientist, it’s not a problem. But if you’re an artist, you’re in trouble already.

The key is that we would be mixing light, not paint. Mixing the three primary colors of light gives white light. If you took three light bulbs (red, green, and blue) and shined them on the ceiling, you’d see white. And if you could magically un-mix the white colors, you’d get the rainbow (which is exactly what prisms do.)

If you’re thinking yellow should be a primary color – it is a primary color, but only in the artist’s world. Yellow paint is a primary color for painters, but yellow light is actually made from red and green light. (Easy way to remember this: think of Christmas colors – red and green merge to make the yellow star on top of the tree.)

As a painter, you know that when you mix three cups of red, green, and blue paint, you get a muddy brown. But as a scientist, when you mix together three cups of cold light, you get white. If you pass a beam white light through a glass filled with water that’s been dyed red, you’ve now got red light coming out the other side. The glass of red water is your filter. But what happens when you try to mix the different colors together?

The cold light is giving off its own light through a chemical reaction called chemiluminescence, whereas the cups of paint are only reflecting nearby light. It’s like the difference between the sun (which gives off its own light) and the moon (which you see only when sunlight bounces off it to your eyeballs). You can read more about light in our Unit 9: Lesson 1 section.

Here’s what you need:

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disposable clear plastic cup or glass jar
one of each: red, green, and blue true-color light sticks (Order true color glow sticks here.)
scissors with adult help
sink

Download Student Worksheet & Exercises

You can demonstrate the primary colors of light using glow sticks! When red, green, and blue cold light are mixed, you get white light.

Simply activate the light stick (bend it until you hear a *crack* – that’s the little glass capsule inside breaking) and while wearing gloves, carefully slice off one end of the tube with strong cutters, being careful not to splash (do this over a sink).

Cut off the ends for all three light sticks. Pass the contents of the light sticks through a coffee filter (or paper towel) into a disposable cup – this will capture the glass bits. Now your cup should be glowing white.

Sometimes the chemical light sticks contain a glowing green liquid encapsulated within a red or blue plastic tube, so when you slice it open to combine it with the other colors, it isn’t a true red. Be sure that your chemical light sticks contain a glowing RED LIQUID and BLUE LIQUID in a clear, colorless plastic tube, or this experiment won’t work. Order true color glow sticks here.

Exercises

What color do you get when you mix blue and green liquid lights?
What happens when you start to add the red light?
What is your final color result when mixing red, blue, and green lights?
How would your result differ if you instead mixed red, blue and green paints?

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Measuring the Speed of Light with a Chocolate Bar

Wednesday February 3, 2010

When you warm up leftovers, have you ever wondered why the microwave heats the food and not the plate? (Well, some plates, anyway.) It has to do with the way microwave ovens work.

Microwave ovens use dielectric heating (or high frequency heating) to heat your food. Basically, the microwave oven shoots light beams that are tuned to excite the water molecule. Foods that contain water will step up a notch in energy levels as heat. (The microwave radiation can also excite other polarized molecules in addition to the water molecule, which is why some plates also get hot.)

One of the biggest challenges with measuring the speed of light is that the photons move fast… too fast to watch with our eyeballs. So instead, we’re going to watch the effects of microwave light and base our measurements on the effects the light has on different kinds of food. Microwaves use light with a wavelength of 0.01 to 10 cm (that’s ‘microwave’ part of the electromagnetic spectrum). When designing your experiment, you’ll need to pay close attention to the finer details such as the frequency of your microwave oven (found inside the door), where you place your food inside the oven, and how long you leave it in for.

Materials:

chocolate bar (extra-large bars work best)
microwave
plate
ruler
calculator
pencil and paper

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Download Student Worksheet & Exercises

First, you’ll need to find the ‘hot spots’ in your microwave. Remove the turntable from your microwave and place a naked bar of chocolate on a plate inside the microwave. Make sure the chocolate bar is the BIG size – you’ll need at least 7 inches of chocolate for this to work. Turn the microwave on and wait a few minutes until you see small parts of the chocolate bar start to bubble up, and then quickly open the door (it will start to smoke if you leave it in too long). Look carefully at the chocolate bar without touching the surface… you are looking for TWO hotspots, not just one – they will look like small volcano eruptions on the surface of the bar. If you don’t have two, grab a fresh plate (you can reuse the chocolate bar) and try again, changing the location of the place inside the microwave. You’re looking for the place where the microwave light hits the chocolate bar in two spots so you can measure the distance between the spots. Those places are the places where the microwave light wave hits the chocolate.

Open up the door or look on the back of your microwave for the technical specifications. You’re looking for a frequency in the 2,000-3,000 MHz range, usually about 2450 MHz. Write this number down on a sheet of paper – this tells you the microwave radiation frequency that the oven produces, and will be used for calculating the speed of light. (Be sure to run your experiment a few times before taking actual data, to be sure you’ve got everything running smoothly. Have someone snap a photo of you getting ready to test, just for fun!)

When you’re ready, pop in the first food type on a plate (without the turntable!) into the best spot in the microwave, and turn it on. Remove when both hotspots form, and being careful not to touch the surface of the food, measure the center-to-center distance using your ruler in centimeters.

TIP: If you’re using mini-marshmallows or chocolate chips (or other smaller foods), you’ll need to spread them out in an even layer on your plate so you don’t miss a spot that could be your hotspot!

How to Calculate the Speed of Light from your Data

Note that when you measure the distance between the hotspots, you are only measuring the peak-to-peak distance of the wave… which means you’re only measuring half of the wave. We’ll multiply this number by two to get the actual length of the wave (wavelength). If you’re using centimeters, you’ll also need to convert those to meters by dividing by 100.

So, if you measure 6.2 cm between your hotspots, and you want to calculate the speed of light and compare to the published value which is in meters per second, here’s what you do:

2,450 MHz is really 2,450,000,000 Hz or 2,450,000,000 cycles per 1 second

Find the length of the wave (in cm): 2 * 6.2 cm = (12.4 cm) /(100 cm/m) = 0.124 meters

Multiply the wavelength by the microwave oven frequency:

0.124 m * 2,450,000,000 Hz = 303,800,000 m/s

Published value for light speed is 299,792,458 m/s = 186,282 miles/second = 670,616,629 mph

Click here to learn how to turn this project into a Science Fair Project you can enter!

Exercises

What would happen if you used cheese instead of chocolate?
Does it matter where in the microwave the chocolate is located? Does placement of the chocolate affect the wavelength?
Can you explain what the burn marks on the chocolate bar are from?

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Rainbow Shadows

Wednesday February 3, 2010

Imagine you’re a painter. What three colors do you need to make up any color in the universe? (You should be thinking: red, yellow, and blue… and yes, you are right if you’re thinking that the real primary colors are cyan, magenta, and yellow, but some folks still prefer to think of the primary colors as red-yellow-blue… either way, it’s really not important to this experiment which primary set you choose.)

Here’s a trick question – can you make the color “yellow” with only red, green, and blue as your color palette? If you’re a scientist, it’s not a problem. But if you’re an artist, you’re in trouble already.

The key is that we would be mixing light, not paint. Mixing the three primary colors of light gives white light. If you took three light bulbs (red, green, and blue) and shined them on the ceiling, you’d see white. And if you could magically un-mix the white colors, you’d get the rainbow (which is exactly what prisms do.)

If you’re thinking yellow should be a primary color – it is a primary color, but only in the artist’s world. Yellow paint is a primary color for painters, but yellow light is actually made from red and green light. (Easy way to remember this: think of Christmas colors – red and green merge to make the yellow star on top of the tree.) It’s because you are using projection of light, not the subtrative combination of colors to get this result.

Here’s a nifty experiment that will really bring these ideas to life (and light!):

Materials:

flashlight (three is best, but you can get by with two)
fingernail polish (red, green, and blue)
clear tape or cellophane (saran wrap works too)
white wall space

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Download Student Worksheet & Exercises

Here’s what you do: Find three flashlights. Cover each with colored cellophane (color filters) or paint the plastic lens cover with nail polish (red, green, and blue). Shine onto a white ceiling or wall, overlap the colors and make new colors. Leave the flashlights on, line them up on a table, turn off the lights, and dance – you will be making rainbow shadows on the wall! In addition, you can paint the lens of a fourth flashlight yellow to see what happens.

When you combine red and green light, you will get yellow light. Combine green and blue to get cyan (turquoise). Combine blue and red to get magenta (purple). Turn on the red and green lights and the wall will appear yellow. Wave your hand in front of the lights and you will see cyan and magenta shadows. Turn on the green and blue lights, and the wall turns cyan with yellow and magenta shadows. Turning on the blue and red give a magenta wall with yellow and cyan shadows. Turn on all colors and you will get a white wall with cyan, yellow, and magenta shadows – rainbow shadows!

Troubleshooting: This experiment has a few things to be aware of. If you’re not getting the colored shadows, check to be sure that the flashlight is bright enough to illuminate a wall in the dark. Be sure to shut the doors, shades, windows, and drapes. In the dark, when you shine your red flashlight on the wall, the wall should glow red. Beware of using off-color nail polish – make sure it’s really red, not hot pink.

If you still need help making this experiment work, you can visit your local hardware store and find three flood lamp holders (the cheap clamp-style ones made from aluminum work well – you’ll need three) and screw in colored “party lights” (make one red, one green, and one blue), which are colored incandescent bulbs. These will provide a lot more light! You can also add a fourth yellow light to further illustrate how yellow light isn’t a primary color. Try using only red, yellow, and blue… you’ll quickly find that you can’t obtain all the colors as you could with the original red-green-blue lights.

Exercises

What are the three primary colors of light?
What color do you get when mixing the primary colors of light?
How do you mix the primary colors of light to get yellow?
Use crayons or colored pencils to draw what you saw when all three lights were shining on the wall and you waved your hand in front of the light.

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Light, Lasers, and Optics

Wednesday February 3, 2010

When I was in grad school, I needed to use an optical bench to see invisible things. I was trying to ‘see’ the exhaust from a new kind of F15 engine, because the aircraft acting the way it shouldn’t – when the pilot turned the controls 20^o left, the plane only went 10^o. My team had traced the problem to an issue with the shock waves, and it was my job to figure out what the trouble was. (Anytime shock waves appear, there’s an energy loss.)

Since shock waves are invisible to the human eye, I had to find a way to make them visible so we could get a better look at what was going on. It was like trying to see the smoke generated by a candle – you know it’s there, but you just can’t see it. I wound up using a special type of photography called Schlieren.

An optical table gives you a solid surface to work on and nails down your parts so they don’t move. This is an image taken with Schlieren photography. This technique picks up the changes in air density (which is a measure of pressure and volume).

The air above a candle heats up and expands (increases volume), floating upwards as you see here. The Schlieren technique shines a super-bright xenon arc lamp beam of light through the candle area, bounces it off two parabolic mirrors and passes it through a razor-edge slit and a neutral density filter before reaching the camera lens. With so many parts, I needed space to bolt things down EXACTLY where I wanted them. The razor slit, for example, just couldn’t be anywhere along the beam – it had to be right at the exact point where the beam was focused down to a point.

I’m going to show you how to make a quick and easy optical lab bench to work with your lenses. Scientists use optical benches when they design microscopes, telescopes, and other optical equipment. You’ll need a bright light source like a flashlight or a sunny window, although this bench is so light and portable that you can move it to garage and use a car headlight if you really want to get creative. Once your bench is set up, you can easily switch out filters, lenses, and slits to find the best combination for your optical designs. Technically, our setup is called an optical rail, and the neat thing about it is that it comes with a handy measuring device so you can see where the focal points are for your lenses. Let’s get started:
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Materials:

lenses (glass or plastic), magnifying lenses work also
two razor blades (new)
index cards (about four)
razor
old piece of wood
single hair from your head
tape
aluminum foil
clothespins (2-4)
laser pointer
popsicle sticks (tongue-depressor size)
hot glue gun
scissors and a sharp razor
meter sticks (2)
bright light source (ideas for this are on the video)

Your lenses are curved pieces of glass or plastic designed to bend (refract) light. A simple lens is just one piece, and a compound lens is like the lens of a camera – there’s lots of them in there. The first lenses were developed by nature – dewdrops on plant leaves are natural lenses. The light changes speed and bends when it hits the surface of the drop, and things under the drop appear larger. (Read more about refraction here.) The earliest written records of lenses are found in the Greek archives and described as being glass globes filled with water.

Concave Lenses

Concave lenses are shaped like a ‘cave’ and curve inward like a spoon. Light that shines through a concave lens bends to a point (converging beam). Ever notice how when you peep through the hole in a door (especially in a hotel), you can see the entire person standing on the doorstep? There’s a concave lens in there making the person appear smaller.

You’ll also find these types of lenses in ‘shoplifting mirrors’. Store owners post these mirrors around help them see a larger area than a flat mirror shows, although the images tend to be a lot smaller.

If you have a pair of near-sighted glasses, chances are that the lenses are concave. Near-sighted folks need help seeing things that are far away, and the concave lenses increase the focal point to the right spot on their retina.

Concave lenses work to make things look smaller, so there not as widely used as convex lenses. You’ll find concave lenses inside camera lenses and binoculars to help clear weird optical problems that happen around the edges of a convex lens (called aberration).

Here’s a video on lenses, both convex and concave:

Convex lenses bulge outwards, bending the light out in a spray (diverging beam). A hand-held magnifying glass is a single concave lens with a handle. These lenses have been used as ‘burning glasses’ for hundreds of years – by placing a small piece of paper at its focal point and using the sun as a light source, you can focus the light energy so intensely that you reach the flash point of the paper (the paper auto-ignites around 450^oF).

When you stack a large convex lens above a solar panel, the magnification effect makes it so you can get away with using a smaller photovoltaic cell to get the same amount of energy from the sun. You’ll find convex lenses in telescopes, microscopes, binoculars, eyeglasses, and more.

Mirrors

What if you coat one side of the lens with a reflecting silver coating? You get a mirror!

Stick wooden skewers into a piece of foam to simulate how the light rays reflect off the surface of the mirror. Note that when the mirror (foam) is straight, the light rays are straight (which is what you see when you look in the bathroom mirror). The light bounces off the straight mirror and zips right back at you, remaining parallel.

Now arch the foam. Notice how the light ways (skewers) come to a point (focal point).

After the focal point, the rays invert, so the top skewer is now at the bottom and the bottom is now at the top. This is your flipped (inverted) image. This is what you’d see when you look into a concave mirror, like the inside of a metal spoon. You can see your face, but it’s upside-down.

Slits

A slit allows light from only one source to enter. If you have light from other sources, your light beam is more scattered and your images and lines become blurry. Thin slits can be easily made by placing the edges of two razor blades very close together and securing into place. We’re going to use an anti-slit using a piece of hair, but you can substitute a thin needle.

Here’s a video on using filters and slits with your laser:

Filters

There are hundreds of different types of filters, used in photography, astronomy, and sunglasses. A filter can change the amount and type of light allowed through it. For example, if you put on red-tinted glasses, suddenly everything takes on a reddish hue. The red filter blocks the rest of the incoming wavelengths (colors) and only allows the red colors to get to your eyeball. There are color filters for every wavelength, even IR and UV.

UV filters reduce the haziness in our atmosphere, and are used on most high-end camera lenses, while IR filters are heat-absorbing filters used with hot light sources (like near incandescent bulbs or in overhead projectors).

A neutral density (ND) filter is a grayish-colored filter that reduces the intensity of all colors equally. Photographers use these filters to get motion blur effects with slow shutter speeds, like a softened waterfall.

Build an Optical Bench

It’s time to put all these pieces together and make cool optical stuff – are you ready?

Download Student Worksheet & Exercises

Click here for more experiments on building your own microscope and telescope.

Cat’s Eyes

Corner reflectors are U-turns for light beams. A corner mirror made from three mirrors will reflect the beam straight back where it came from, no matter what angle you hit it at. Astronauts placed these types of mirrors on the moon so scientists could easily bounce laser beams off the moon and have them return to the same place on Earth. They used these reflected laser beams to measure the speed of light.

You’ll find corner mirrors in “cat’s eye” reflectors on the road. Car headlights illuminate the reflectors and send the beam straight back the same way – right at the driver.

Exercises

Using only the shape, how can you tell the difference between a convex and a concave lens?
Which type of lens makes objects viewed through it appear smaller?
Which type of lens makes the objects viewed through it appear larger?
How do you get the f number?

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Benham’s Disk

Wednesday February 3, 2010

Charles Benhamho (1895) created a toy top painted with the pattern (images on next page). When you spin the disk, arcs of color (called “pattern induced flicker colors”) show up around the disk. And different people see different colors!

We can’t really say why this happens, but there are a few interesting theories. Your eyeball has two different ways of seeing light: cones and rods. Cones are used for color vision and for seeing bright light, and there are three types of cones (red, green, and blue). Rods are important for seeing in low light.

One possibility is… [am4show have=’p8;p9;p19;p46;p66;’ guest_error=’Guest error message’ user_error=’User error message’ ]

…how the human eye is tuned for different colors. Your eyeballs respond at different rates to red, green, and blue colors. The spinning disk triggers different parts of the retina. This alternating response may cause some type of interaction within the nervous system that generates colors.

Another theory is that certain cones take longer react, and thus stay active, for longer amounts of time (though we’re still talking milliseconds, here). To put another way, the white color activates all three cones, but then the black deactivates them in a certain sequence, causing your brain to get mixed and unbalanced signals. Your brain does the best it can to figure it out the information it’s getting, and “creates” the colors you see in order to make sense of it all.

Neither of these theories explains the colors of Benham’s disk completely and the reason behind the illusion remains unsolved. Can you help out these baffled scientists?

Materials:

Download this PDF file
string (about 3 feet)

Download Student Worksheets & Exercises

All you need to do is download this PDF file and cutout a copy of a disc on the page. Then find a way to spin it at high speeds – you can stick a pencil through the center and spin it like a top, thread string through it and pull to rotate (just like the Mixing Colors Experiment), attach to a drill or mixer or electric screwdriver, or slap it on a motor shaft and engage the power. Which works best?

Exercises

What colors were you able to see when the disks were spinning?
How did the different patterns look when they were spun?
How did speed and direction affect what you saw?

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Liquid Prism

Wednesday February 3, 2010

In this experiment, water is our prism. A prism un-mixes light back into its original colors of red, green, and blue. You can make prisms out of glass, plastic, water, oil, or anything else you can think of that allows light to zip through.

What’s a prism? Think of a beam of light. It zooms fast on a straight path, until it hits something (like a water drop). As the light goes through the water drop, it changes speed (refraction). The speed change depends on the angle that the light hits the water, and what the drop is made of. (If it was a drop of mineral oil, the light would slow down a bit more.) Okay, so when white light passes through a prism (or water drop), changes speed, and turns colors. So why do we see a rainbow, not just one color coming out the other side?

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The secret is because the light is made up of different wavelengths, and each gets bent by different amounts when they hit a new material. So one wave changes speed to red, another to yellow, another to green, etc. when the beam hits the prism. And water drops are tiny prisms.

The light passing through a water drop gets refracted twice, not once. The first time is when it enters the water drop, the second when it bounces off the other side of the drop and reflects back through the water drop and out again (some of the light does make it out the other side of the drop, but most of it bounces back). When the light emerges from the water drop, it changes speed again, and presto! You have a rainbow.

Natural rainbows (the ones that you see after it rains) happen when water drops (tiny prisms) in the air are hit by sunlight from behind you at just the right angle (which is relatively a low angle, near the ground). The best rainbows can be seen when half of the sky is darkened with rainclouds and you’re in a clear patch with sun behind you. And guess what? You can even see a nighttime rainbow (called a moonbow), although they’re pretty rare, usually near full moon.

Here’s what you do:

Materials:

mirror
shallow baking dish
water
sunlight

Download Student Worksheet & Exercises

Set a clear tray of water in sunlight. Lean a mirror against an inside edge and adjust so that a rainbow appears on the wall. You can also use a light bulb shining through a slit in a flat cardboard piece as a light source.

Troubleshooting: This is one of the easiest experiments to do, and the most beautiful. The trouble is, you don’t know where the water shadow will show up, so make sure you point the mirror to the sky and play with the angle of the mirror until you find the wavering rainbow. Because the shadow is constantly moving, you can snap a few pictures when you’ve got it so you can look over the finer details later. If this project still eludes you, take a large sheet and use it instead of the tiny index card.

Exercises

What serves as the prism in this experiment?
What property can help make something a good prism material?
What are some other items that could be used as prisms?

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Kaleidoscopes

Wednesday February 3, 2010

In a simplest sense, a kaleidoscope is a tube lined with mirrors. Whether you leave the end opened or tape on a bag of beads is up to you, but the main idea is to provide enough of an optical illusion to wow your friends. Did you know that by changing the shape and size of the mirrors, you can make the illusion 3D?

If you use only two mirrors, you’ll get a solid background, but add a third mirror and tilt together into a triangle (as shown in the video) and you’ll get the entire field filled with the pattern. You can place transparent objects at the end (like marbles floating in water or mineral oil) or just leave it open and point at the night stars.

The first kaleidoscopes were constructed in 1816 by a scientist while studying polarization. They were quickly picked up as an amusement gadget by the public and have stayed with us ever since.

Materials:

three mirrors the same size
tape and scissors

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Download Student Worksheet & Exercises

Here’s what you do: Carefully tape together three identical mirrors, making a triangle-tube with the mirrors on the inside. (You can also use Mylar or silver wrapping paper taped to cardboard instead of mirrors.) Tape all rough edges well and peek through the opening as you walk around.

Variations: By changing the size and shape of the mirrors, you can change the dimensional effect you see. Just be sure to look at the mirror surface, not the opening. You can also make mirrors wider at the bottom and narrower at the top (easier with cardboard mirrors); use four or five mirrors instead of three; change the length of the mirrors; use curved mirrors instead of flat (find curved cardboard from an oatmeal box or carefully cut apart a soda can and tape Mylar or spray with chrome paint from the hardware store).

Exercises

What is a light source?
What is a light reflector?
Sketch an image of something interesting that you were able to see as the light reflected from the multiple surfaces of the kaleidoscope to your eyes:

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Mixing Colors

Wednesday February 3, 2010

There are three primary colors of light are red, green, and blue. The three primary colors of paint are red, yellow, and blue (I know it’s actually cyan, yellow, and magenta, which we’ll get to in more detail later, but for now just stick with me and think of the primary colors of paint as red-yellow-blue and I promise it will all make sense in the end).

Most kids understand how yellow paint and blue paint make green paint, but are totally stumped when red light and green light mix to make yellow light. The difference is that we’re mixing light, not paint.

Lots of science textbooks still have this experiment listed under how to mix light: “Stir together one of red water and one glass of green water (dyed with food coloring) to get a glass of yellow water.” Hmmm… the result I get is a yucky greenish-brown color. What happened?

The reason you can’t mix green and red water to get yellow is that you’re essentially still mixing paint, not light. But don’t take our word for it – test it out for yourself with this super-fast light experiment on mixing colors.

Materials:

pair of scissors
crayons
sharp wood pencil or wood skewer
index cards
drill (optional)

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Download Student Worksheet & Exercises

Here’s what you do: use a cup to outline circles on a sheet of stiff white paper (or manila folders). Stack several blank pages together and cut out multiple circles. Color the circles, push a sharp wooden pencil through a hole in the center, and spin! What color does yellow and blue make? Pink and purple? You can also make a button-spinner to really whirl it around by looping a length of string through two holes in the center of the disk circle.

Troubleshooting: These disks needs to spin rapidly in order to trick your eye into blending the colors. If you have a motor, batteries, and wires lying around, you can use them to spin the disks for you. Simply punch the motor’s shaft through the paper (colored-side up).

Turn the motor on by connecting the power and watch the colors mix! Other alternatives include using a drill, hand-held mixer/beaters (not a Kitchen Aid standard mixer!), electric screwdriver, etc.

Alternate Spinning Method: Want to do this project, but you don’t have enough speed or a motor? You can make a ‘button spinner’ to whirl these things around super-fast. (Did you know this is how the first circular saws were made?)

Attach your disk to a piece of stiff cardboard (index cards are too flimsy), punch out two holes near the center and thread a loop of string through and tie the ends together to make the old-fashioned “spinning disk”. Using a circling motion with your hands, you can twist up the string with the card in the middle and then pull horizontally outwards to untwist it and watch the cardboard whirl and whip around!!

Click here for the Mixing Cold Light experiment!

Exercises

What happens when blue and red are mixed on the spinner?
What happens when red and green are mixed on the spinner?
What colors would you mix to get orange?
What are the primary colors of light, and how do they differ from the primary colors we learn in art class?

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Diffraction

Wednesday February 3, 2010

Ever play with a prism? When sunlight strikes the prism, it gets split into a rainbow of colors. Prisms un-mix the light into its different wavelengths (which you see as different colors). Diffraction gratings are tiny prisms stacked together.

When light passes through a diffraction grating, it splits (diffracts) the light into several beams traveling at different directions. If you’ve ever seen the ‘iridescence’ of a soap bubble, an insect shell, or on a pearl, you’ve seen nature’s diffraction gratings.

Scientist use these things to split incoming light so they can figure out what fuels a distant star is burning. When hydrogen burns, it gives off light, but not in all the colors of the rainbow, only very specific colors in red and blue. It’s like hydrogen’s own personal fingerprint, or light signature.

Astronomers can split incoming light from a star using a spectrometer (you can build your own here) to figure out what the star is burning by matching up the different light signatures.

Materials:

feather
old CD or DVD

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Here’s what you do: Take a feather and put it over an eye. Stare at a light bulb or a lit candle. You should see two or three flames and a rainbow X. Shine a flashlight on a CD and watch for rainbows. (Hint – the tiny “hairs” on the feather are acting like tiny prisms… take your homemade microscope to look at more of the feather in greater detail and see the tiny prisms for yourself!

What happens when you aim a laser through a diffraction grating? Here’s what you do:

Materials:

laser pointer
diffraction grating (you can use an old CD also)

Download Student Worksheet & Excercises

Exercises

Which light source gave the most interesting results?
What happens when you aim a laser beam through the diffraction grating?
How is a CD different and the same as a diffraction grating?
Why does the feather work?

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Electric Eye

Wednesday February 3, 2010

This is a super-cool and ultra-simple circuit experiment that shows you how a CdS (cadmium sulfide cell) works. A CdS cell is a special kind of resistor called a photoresistor, which is sensitive to light.

A resistor limits the amount of current (electricity) that flows through it, and since this one is light-sensitive, it will allow different amounts of current through depends on how much light it "sees".

Photoresistors are very inexpensive light detectors, and you'll find them in cameras, street lights, clock radios, robotics, and more. We're going to play with one and find out how to detect light using a simple series circuit.

Materials:

AA battery case with batteries
one CdS cell
three alligator wires
LED (any color and type)

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Download Student Worksheet & Exercises

Turn this into a super-cool burglar alarm!

Exercises

How is a CdS cell like a switch? How is it not like a switch?
When is the LED the brightest?
How could you use this as a burglar alarm?

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Materials:

AA battery case with batteries
one CdS cell
three alligator wires
LED (any color and type)

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Download Student Worksheet & Exercises

Turn this into a super-cool burglar alarm!

Exercises

How is a CdS cell like a switch? How is it not like a switch?
When is the LED the brightest?
How could you use this as a burglar alarm?

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Black Light Treasure Hunt

Wednesday February 3, 2010

Ever notice how BRIGHT your white t-shirt looks in direct sun? That’s because mom washed with fluorescent laundry soap (no kidding!). The soap manufacturers put in dyes that glow white under a UV light, which make your clothes appear whiter than they really are.

Since light is a form of energy, in order for things to glow in the dark, you have to add energy first. So where does the energy come from? There are are few different ways to do this:

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Materials:

UV black fluorescent light (check shopping list to find out where to get one)
dark evening inside your house

Light bulbs use incandescence, meaning that the tungsten wire inside a light bulb gets so hot that it gives of light. Unfortunately, bulbs also give off a lot of heat, too. Incandescence happens when your electric stove glows cherry red-hot. Our sun gives off energy through incandescence also – a lot of it.

On the other end of things, cold light refers to the light from a glow stick, called luminescence. A chemical reaction (chemiluminescence) starts between two liquids, and the energy is released in the form of light. On the atomic scale, the energy from the reaction bumps the electron to a higher shell, and when it relaxes back down it emits a photon of light.

Phosphorescence light is the ‘glow-in-the-dark’ kind you have to ‘charge up’ with a light source. This delayed afterglow happens because the electron gets stuck in a higher energy state. Lots of toys and stick-on stars are coated with phosphorescent paints.

Triboluminescence is the spark you see when you smack two quartz crystals together in the dark. Other minerals spark when struck together, but you don’t have to be a rock hound to see this one in action – just take a Wint-O-Green lifesaver in a dark closet with a mirror and you’ll get your own spark show. The spark is basically light from friction.

Fluorescence is what you see on those dark amusement-park rides that have UV lights all around to make objects glow. The object (like a rock) will absorb the UV light and remit a completely different color. The light strikes the electron and bumps it up a level, and when the electron relaxed back down, emits a photon.

Download Student Worksheet & Exercises

Whew! There’s a lot to know about glow in the dark stuff, isn’t there? Let’s pull all this together and go on a Treasure Hunt. This hunt is best done just before bed, when it’s already dark outside. All you need is 20 minutes and a UV black light. Ready?

Here’s what you do: Shut off all the lights in the house and go around armed with your UV light, finding things that glow both inside and outside the house. I’ve found some surprises, including a batch of screaming yellow masking tape, eye-popping orange near the microwave (someone’s spillover from lunch?), and garishly green rocks just outside. Our teeth, laundry soap, and sneakers were way fun, too! What fluoresces in your house? Have fun!

What kind of stuff glows under a black light?

Loads of stuff! There are a bunch of everyday things that fluoresce (glow) when under a black light. Note – black lights emit UV light, some of which you can’t see (just like you can’t see infrared – the beam emitted from the remote control to the TV). By the way, that’s why “black lights” were named as such. The reason stuff glows is that fluorescent objects absorb the UV light and then spit it back almost instantaneously. Some of that energy gets lost during that process, and that changes the wavelength of the light, which makes this light visible and causes the material to appear to ‘glow’.

Here are some things that glow: white paper (although paper made pre-1950 doesn’t, which is how investigators tell the difference between originals and fakes), club soda or tonic water (it’s the quinine that glows blue), body fluids (yes, blood, urine, and more are all fluorescent), Vitamins (Vitamin A, B, B-12 (crush and dissolve in vinegar first), thiamine, niacin, and riboflavin are strongly fluorescent), chlorophyll (grind spinach in a small amount of alcohol (vodka) and pour it through a coffee filter to get the extract (keep the solids in the filter, not the liquid)), antifreeze, laundry detergents, tooth whiteners, postage stamps, driver’s license, jellyfish, and certain rocks (fluorite, calcite, gypsum, ruby, talc, opal, agate, quartz, amber) and the Hope Diamond (which is blue in regular light, but glows red).

When you purchase your UV fluorescent “black lights”, be sure to get the LONG WAVE version (short wave UV is the kind that causes permanent damage to living things – that’s how they kill the bacteria in water), and it appears that UV fluorescent lamps work better than the UV LEDs currently on the market, usually sold as “UV Flashlights”. We tried both, and the stuff shone brighter with the fluorescent lamps.

Exercises

Why are incandescent lights less energy-efficient than fluorescent lights?
What are the two types of fluorescent lights?
What kinds of things did you find that glow on your treasure hunt? Give at least five examples.

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Advanced Telescope Building

Wednesday February 3, 2010

So you’ve played with lenses, mirrors, and built an optical bench. Want to make a real telescope? In this experiment, you’ll build a Newtonian and a refractor telescope using your optical bench.

Materials:

optical bench
index card or white wall
two double-convex lenses
concave mirror
popsicle stick
mirror
paper clip
flash light
black garbage bag
scissors or razor
rubber band
wax paper
hot glue

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Download Student Worksheet & Exercises

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Unit 9: Light Video

Wednesday February 3, 2010

Light can be either a wave or a particle, but not both at the same time. Which one it is will depend on what it’s doing. In this section, you’ll be able to figure out how intensity (how bright), frequency (wavelength), polarization (the direction of the electric field), and phase (time shift) all affect the kind of light you see and don’t see. Most light isn’t detectable by the human eye, which makes studying light more like investigating a crime scene. You’ll quickly be puzzling the pieces together to explain why pencils in a glass of water appear broken, why light beams can fry an egg, and how to build a telescope.

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Fun with UV

Tuesday February 2, 2010

UV (ultra-violet) light is invisible, which means you need more than your naked eyeball to do experiments with it. Our sun gives off light in the UV. Too much exposure to the sun and you’ll get a sunburn from the UV rays.

There are many different experiments you can do with UV detecting materials, such as color-changing UV beads and UV nail polish.

Here are a few fun activities you can do with your UV detecting materials:

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Materials:

UV beads
sunblock
sunglasses
sunlight
clear plastic bag

Testing Sunblock You can test how effective your sunblock is at stopping harmful rays by slapping a coat of the lotion (or SPF-rated lip balm) on the beads and leaving them out in the sun for a minute. Bring the bead indoors and wipe off… did it change color? If so, then UV rays made it through the sunblock to your bead, and chances are that your sunblock isn’t doing it’s job. Does it matter how thick of a coat you layer on?

You can alternatively place the beads inside a plastic bag and coat the outside of the bag with sunblock. And is the sunblock really waterproof? Meaning that they still are white after dunking the beads underwater while sunblocked…?!

Different Times of Day Stick the UV beads outside and take note how bright the colors are in the morning, noon, and afternoon. You’ll notice a big difference depending on the sun’s spot in the sky. Does it matter whether it’s sunny or cloudy?

Absorption and Filtration Test out different lenses and filters to see which block UV light. Lay a handful of beads on the sidewalk and set a pair of sunglasses in top (lenses sitting on the layer of beads). Did any UV light make it through? If it didn’t, your beads should stay white. What other things can you test? (Hint: how about inside your car?)

Download Student Worksheet & Exercises

Why does that work? UV sensitive materials have a pigment inside that changes color when exposed to UV light from either the sun or lights that emit in the 350nm – 300nm wavelength. (UVA is high-energy: 400-320nm, and UVB is low energy: 320-280nm). If you have fluorescent black lights, use them. (Do regular incandescent bulbs work? If not, you know they emit light outside the range of the beads!)

When light hits the pigment molecule, it absorbs the energy and actually expands asymmetrically (one end of the molecule expands more than the other). Different expansion amounts will give you a different color. Although it’s a bit more complicated that that, you now have the basic idea. Your beads will change colors thousands of times before they wear out, so enjoy these super-inexpensive UV detectors!

Exercises

What kinds of light sources didn’t work with the UV beads?
Did your sun block really block out the UV rays?
Which was the best protection against UV rays?

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Quantum Mechanics: Double Slit Experiment

Sunday December 13, 2009

This stuff is definitely sci-fi weird, and probably not appropriate for younger grades (although we did have a seven year old reiterate in his own words this exact phenomenon to a physics professor, so hey… anything possible! Which is why we’ve included it here.)

This experiment is also known as Young’s Experiment, and it demonstrates how the photon (little packet of light) is both a particle and a wave, and you really can’t separate the two properties from each other. If the idea of a ‘photon’ is new to you, don’t worry – we’ll be covering light in an upcoming unit soon. Just think of it as tiny little packets or particles of light. I know the movie is a little goofy, but the physics is dead-on. Everything that “Captain Quantum” describes is really what occurred during the experiment. Here’s what happened:

So basically, any modification of the experiment setup actually determines which slit the electrons go through. This experiment was originally done with light, not electrons. and the interference pattern was completely destroyed (as shown in the end of the video) by an ‘observer’. This shows you that light can either be a wave or a particle, but not both at the same time, and it has the ability to flip between one and the other very quickly. (The image at the left is a photograph of an interference pattern – the same thing you’d see on the wall if you tried this experiment.)

So, both light and electrons have wave-particle characteristics. Now, take your brain this last final step… it’s easier to see how this could be true for light, you can imagine as a massless photon.

But an electron has mass. Which means that matter can act as a wave. Twilight zone, anyone?

Read more about this in our Advanced Physics Section.

Advanced High School Course in Radio Astronomy

Sunday November 22, 2009

Radio antenna dishes of the Very Large Array radio telescope in New Mexico.

This experiment is for advanced students. Radio astronomy is the study of radio waves originating outside the Earth. The radio range of frequencies or wavelengths is loosely defined by three factors: atmospheric transparency, current technology, and fundamental limitations imposed by quantum noise. Together they yield a boundary between radio and far-infrared astronomy at frequency 1 THz (1 THz =1012 Hz) or wavelength =0.3 mm.

If you’re an advanced high school-age student with a yearning to learn more about radio astronomy, you’re in the right place. First, you’ll get a college-level course about the fundamentals of radio astronomy with a full textbook, and you’ll also find problem sets with solutions and also a final exam.The lab included will have you building your very own radio telescope for under $100. Feel free to build the telescope as you work through the text or straight off the bat. If you’re allergic to math, just skip over those sections to get at the really interesting stuff. Click here to download the full text or use the links below.

Microwaving Soap

Friday October 2, 2009

When you warm up leftovers, have you ever wondered why the microwave heats the food and not the plate? (Well, some plates, anyway.) It has to do with the way microwaves work.

Microwaves generate high energy electromagnetic waves that when aimed at water molecules, makes these molecules get super-excited and start bouncing around a lot.

We see this happen when we heat water in a pot on the stove. When you add energy to the pot (by turning on the stove), the water molecules start vibrating and moving around faster and faster the more heat you add. Eventually, when the pot of water boils, the top layer of molecules are so excited they vibrate free and float up as steam.

When you add more energy to the water molecule, either by using your stove top or your nearest microwave, you cause those water molecules to vibrate faster. We detect these faster vibrations by measuring an increase in the temperature of the water molecules (or in the food containing water). Which is why it’s dangerous to heat anything not containing water in your microwave, as there’s nowhere for that energy to go, since the electromagnetic radiation is tuned to excite water molecules.

To explain this to younger kids (who might confuse radio waves with sounds waves) you might try this:

There’s light everywhere, some of which you can see (like rainbows) and others that you can’t see (like the infrared beam coming from your TV remote, or the UV rays from the sun that give you a sunburn). The microwave shoots invisible light beams at your food that are tuned to heat up the water molecule.

The microwave radiation emitted by the microwave oven can also excite other polarized molecules in addition to the water molecule, which is why some plates also get hot. The soap in this experiment below will show you how a bar of Ivory soap contains air, and that air contains water vapor which will get heated by the microwave radiation and expand. Are you ready?

Here’s what you need:

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bar of Ivory soap
microwave (not a new or expensive one)
plate (optional)

The following experiment is a quick example of this principle using a naked bar of Ivory soap. The trick is to use Ivory, which contains an unusually high amount of air. Since air contains water moisture, Ivory also has water hidden inside the bar of soap. The microwave will excite the water molecules and your kids will never look at the soap the same way again.

Download Student Worksheet & Exercises

Toss a naked bar of Ivory soap onto a glass or ceramic plate and stick it into the microwave (don’t use a new or expensive microwave!) on HIGH for 2-3 minutes. Watch intently and remove when it reaches a “maximum”. Be careful when you touch it after taking it out of the microwave oven – it may still hold steam inside. You can still use the soap and the microwave after this experiment!

Note: Scientists refer to ‘light’ as the visible part of the electromagnetic spectrum, where radio and microwaves are lower energy and frequency than light (and the height of the wave can be the size of a football field). Gamma rays and x-rays are higher energy and frequency than light (these tend to pass through mirrors rather than bounce off them. More on that in Unit 9.)

Exercises

What is it in your food (and the soap) that is actually heated by the microwave?
How does a microwave heat things?
Touch the soap after it has been allowed to cool for a few minutes and record your observations.

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Key Concepts

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Questions to Ask

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Crazy Remote

Making IR Visible to the Human Eye

Double Your Money

How to Make a Microscope

How to Make a Telescope

How to Calculate the Speed of Light from your Data

Concave Lenses

Mirrors

Slits

Filters

Build an Optical Bench

Cat’s Eyes

What kind of stuff glows under a black light?