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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  1. 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.
  2. Rotate that polarizer 90o. Does the light intensity (brightness) change? (It should not.)
  3. 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.)
  4. Rotate one of the polarizers 90o 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 90o and blocks all the light from the computer screen.
  5. 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 90o 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 45o 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 45o 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 45o 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 90o 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

  1. Can light change speeds?
  2. Can you see ALL light with your eyes?
  3. Give three examples of a light source.
  4. 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?
  5. What if you use oil instead of water for bending a pencil?
  6. How does a microscope work?
  7. What's the difference between a microscope and a telescope?
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What is a Laser?

Thursday March 20, 2014

Have you ever wondered why you just can’t just shine a flashlight through a lens and call it a laser? It’s because of the way a laser generates light in the first place.

The word LASER is an acronym for Light Amplification by Stimulated Emission of Radiation.
That’s a mouthful. Let's break it down.

Let's do an experiment that shows you how a laser is different from light from a flashlight by looking at the wavelengths that make up the light.

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Materials:

  • laser such as these: https://amzn.to/3FAog5f
  • diffraction grating or old CD
  • flashlight
  • clear tape
  • red, green, and/or blue fingernail polish



Download your student worksheet here! This download was provided by Laser Classroom. 

Lasers are optical light that is amplified, which means that you start with a single particle of light (called a photon) and you end up with a lot more than one after the laser process.

Stimulated emission means that the atom you’re working with, which normally hangs out at lower energy levels, gets excited by the extra energy you’re pumping in, so the electrons jump into a higher energy level. When a photon interacts with this atom, if the photon as the same exact energy as the jump the electron made to get to the higher level, the photon will cause the electron to jump back down to the lower level and simultaneously give off a photon in the same exact color of the photon that hit the atom in the first place.

The end result is that you have photons that are the same color (monochromatic) and in synch with each other. This is different from how a light bulb creates light, which generates photons that are scattered, multi-colored, and out of phase. The difference is how the light was generated in the first place.

Radiation refers to the incoming photon. It’s a word that has a bad connotation to it (people tend to think all radiation is dangerous, when really it’s only a small percentage that is). So in this case, it just means light in the laser. The incoming photon radiation that starts the process of stimulated emission (when the electron jumps between energy levels and generates another photon), and the light amplification means that you started with one photon, and you ended up with two. Put it all together and you have a LASER!

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Laser Collimation

Thursday March 20, 2014

Laser light is collimated, meaning that it travels in parallel rays. Here’s a really cool experiment that will show you the difference between a non-collimated light, like from a flashlight and collimated light from a laser.


Ordinary light from a light bulb diverges as it travels. It spreads out and covers a larger and larger area the further out you go. A laser has little to no divergence, so we way that laser light is collimated.


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Materials:


  • flashlight
  • laser
  • ruler
  • pencil
  • piece of paper


 


Download your student worksheet here!


This is a quick overview of what a laser is, and why you can’t make a laser from a flashlight beam.


  1.  Cover the end of the flashlight with clear tape. Paint the tape with red nail polish.
  2. Stand close to the wall and shine both the laser and the flashlight at the wall.
  3. Now slowly move backwards. What happens to the laser and the flashlight light on the wall?

The laser dot doesn’t change size (or if it does, it’s not much), but the pool of light from the flashlight increases in size.   The light from the laser travels in the same direction in a straight line, called collimated light. The flashlight beam diverges, or spreads out.


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Laser Coherence

Thursday March 20, 2014

Lasers light is different from light from a flashlight in a couple of different ways. Laser light is monochromatic, meaning that it’s only one color.


Laser light is also coherent, which means that the light is all in synch with each other, like soldiers marching in step together. Since laser light is coherent, which means that all the light waves peaks and valleys line up. The dark areas are destructive interference, where the waves cancel each other out. The areas of brightness are constructive interference, where the light adds, or amplifies together. LED light is not coherent because the light waves are not in phase.


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Materials:


  • laser
  • flashlight


 


Download your student worksheet here! This download is provided by Laser Classroom. Check out their website for more free downloads and really cool lasers!


Hold your flashlight very, very close to a sheet of paper at a small angle and look at the light on the paper. Do you see any dark spots, or is it all the same brightness? (It should be the same brightness.)


Now try this with a red laser (do NOT use a green laser). Hold it very close to the paper again at a small angle and look for tiny dark spots, like speckles. Those are coherent waves interfering with each other. It’s really hard to see this, so you may not be able to find it with your eyes. (You can pass the light through a filter (like a gummy bear) to cut down on the intensity so the speckle pattern shows up better.)


What’s happening is this: light travels in waves, and when those waves are in phase (coherent) they interfere with each other in a special way. They cancel each other out (destructive interference) or amplify (constructive interference). This pattern isn’t found with sunlight or light from a bulb because that kind of light all out of phase and doesn’t have this kind of distinct interference pattern.


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Measuring the Wavelength of a Laser

Thursday March 20, 2014

Diffraction is how light bends as it passes through very narrow slits or around very thin objects like a hair. When light travels around a hair, two wave patterns form, and those waves interfere with each other constructively (they add together to form a bright region) or destructively (the cancel each other out and leave a dark spot).


This experiment looks at the light and dark areas of interference to determine the wavelength of a laser. You can do this for lasers that don’t have labels on them, so you really don’t know what wavelength they are!


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Materials:


  • laser
  • diffraction grating
  • calculator
  • blank wall
  • ruler/yardstick


Download your student worksheet here! This download is provided by Laser Classroom. Check out their website for more free downloads and really cool lasers!


The math for figuring this out is easy. You first need to know the distance between the slits (d) and how far apart the two maximums are as shown in the video (X), and multiply those two together. If you make your diffraction grating 1 meter from the wall, you divide your product by 1 meter to get your wavelength. Watch your units or you’ll be way off in your calculations!


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Gummy Bears, Absorption, and Transmittance

Thursday March 20, 2014

Gummy bears are a great way to bust one of the common misconceptions about light reflection. The misconception is this: most students think that color is a property of matter, for example if I place shiny red apple of a sheet of paper in the sun, you’ll see a red glow on the paper around the apple.


Where did the red light come from? Did the apple add color to the otherwise clear sunlight? No. That’s the problem. Well, actually that’s the idea that leads to big problems later on down the road. So let’s get this idea straightened out.


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Materials:


  • flashlight
  • laser
  • red and green gummy bear


Download your student worksheet here! This download is provided by Laser Classroom. Check out their website for more free downloads and really cool lasers!


It’s really hard to understand that when you see a red apple, what’s really happening is that most of the wavelengths that make up white light (the rainbow, remember?) are absorbed by the apple, and only the red one is reflected. That’s why the apple is red.


When the light hits something, it gets absorbed and either converted to heat, reflected back like on a mirror, or transmitted through like through a window.


When you shine your flashlight light through the red gummy bear, the red gummy is acting like a filter and only allowing red light to pass through, and it absorbs all the other colors. The light coming from out the back end of the gummy bear is monochromatic, but it’s not coherent, not all lined up or in synch with each other. What happens if you shine your flashlight through a green gummy bear? Which color is being absorbed or not absorbed now?


Now remember, the gummy bear does NOT color the light, since white light is made up of all visible colors, red and green light were already in there. The red gummy bear only let red through and absorbed the rest. The green gummy bear let green through and absorbed the rest.


Now…take out your laser. There’s only one color in your laser, right? Shine your laser at your gummy bears. Which gummy bear blocks the light, and which lets it pass through?  Why is that? I’ll give you one minute to experiment with your gummy bears and your lasers.


In the image above, the two on the left are green gummy bears, and the two on the right are red gummy bears. The black thing is a laser. The dot on the black laser tells you what color the laser light is, so the laser on the far left is a red laser shining on a green gummy bear. Do you see how the light is really visible out the back end of the gummy bear in only two of the pictures? What does that tell you about light and how it gets transmitted through an object?


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Lasers, Jell-O and Trigonometry

Thursday March 20, 2014

If you’re scratching your head during math class, wondering what you’ll ever use this stuff for, here’s a cool experiment that shows you how scientists use math to figure out the optical density of objects, called the “index of refraction”.


How much light bends as it goes through one medium to another depends on the index of refraction (refractive index) of the substances. There are lots of examples of devices that use the index of refraction, including fiber optics. Fiber optic cables are made out of a transparent material that has a higher index of refraction than the material around it (like air), so the waves stay trapped inside the cable and travel along it, bouncing internally along its length.  Eyeglasses use lenses that bend and distort the light to make images appear closer than they really are.
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Materials:


  • Paper
  • Laser
  • Pencil
  • Protractor
  • Ruler
  • Gelatin (1 box)
  • 1/2 cup sugar
  • 2 containers
  • Hot (boiling) water with adult help
  • Knife with adult help


Download Student Worksheet & Exercises


 Experiment:


  1. Mix two packets of gelatin with one cup of boiling water and stir well.
  2. To one of the containers, add 1/2 cup sugar. Label this one as “sugar” and put the lid on and store it in the fridge.
  3. Label the other as “plain” and also store it in the fridge. It takes about 2 hours to solidify. Wait, and then:
  4. Cut out a 3”x3” piece of gelatin from the plain container.
  5. On your sheet of paper, mark a long line across the horizontal, and then another line across the vertical (the “normal” line) as shown in the video.
  6. Mark the angle of incidence of 40o. This is the path your laser is going to travel on.
  7. Lay down the gelatin so the bottom part is aligned with the horizontal line.
  8. Shine your laser along the 40o angle of incidence. Make sure it intersects the origin.
  9. Measure the angle of refraction as the angle between the bent light in the gelatin and the normal line. (It’s 32o in the video.)
  10. Use Snell’s Law to determine the index of refraction of the gelatin: n1 sin θ1 = n2 sin θ2
  11. Repeat steps 4-10 with the sugar gelatin. Did you expect the index of refraction to be greater or less than the plain version, and why?

 Questions to Ask:


  1. Does reflection or refraction occur when light bounces off an object?
  2. Does reflection or refraction occur when light is bent?
  3. What type of material is used in a lens?
  4. What would happen if light goes from air to clear oil?

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Reflection Law using a Laser

Thursday March 20, 2014

The angle that the reflected light makes with a line perpendicular to to the mirror is always equal to the angle of the incident ray for a plane (2-dimensional) surface.


We’re going to play with how light reflects off surfaces. At what angle does the light get reflected? This experiment will show you how to measure it.


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Materials:


  • laser
  • mirror
  • protractor
  • pencil
  • paper


 


These downloads are provided by Laser Classroom. Check out their website for more free downloads and really cool lasers!


Click here for the chapter in optics for advanced students.


Did you notice a pattern? When the laser beam hits the mirror at a 30o angle, it comes off the mirror at 60o, which means that the angle on both sides of a line perpendicular to the mirror are equal. That’s the law of reflection on a plane surface.


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Refraction using Lasers and Water

Thursday March 20, 2014

This simple activity has surprising results! We’re going to bend light using plain water. Light bends when it travels from one medium to another, like going from air to a window, or from a window to water. Each time it travels to a new medium, it bends, or refracts. When light refracts, it changes speed and wavelength, which means it also changes direction.


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Materials:


  • Red and green laser
  • Paperclip
  • Index card
  • Tape
  • Rubber band
  • Water glass


  1. Open the paperclip into an “L” shape, and tape it to an index card so the card stands up. This is your projection screen.
  2. Use the rubber band to attach the laser pointers together. You’ll want them very close and parallel to each other. Place the rubber band close to the ON button so the laser will stay on when you put the rubber band over it.
  3. Place the laser pointers on a stack of books and put switch them on with the rubber band.
  4. Shine the lasers through the middle of an empty glass jar and onto the screen.
  5. Put a mark where the red and green laser dots are on the screen.
  6. With the lasers still on, slowly fill the container with water. What happened to the dots?
  7. You can add a couple of drops of milk or a tiny sprinkling of cornstarch to the water to see the beams in the water.

Here’s a quick activity you can do if the idea of refraction is new to you… Take a perfectly healthy pencil and place it in a clear glass of water.  Did you notice how your pencil is suddenly broken? What happened? Is it defective? Optical illusion?  Can you move your head around the glass in all directions and find the spot where the pencil gets fixed? Where do you need to look to see it broken?


When light travels from water to air, it bends. The amount it bends is measured by scientists and called the index of refraction, and it depends on the optical density of the material. The more dense the water, the slower the light moves, and the greater the light gets bent. What do you think will happen if you use cooking oil instead of water?


So the idea is that light can change speeds, and  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 couple of values for you to think about:


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, because it’s got the same index of refraction! 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.


Questions to Ask


1. Is there a viewing angle that makes the pencil whole?


2. Can we see light waves?


3. Why did the green and red laser dots move?


4. What happens if you use an optically denser material, like oil?
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Laser Safety

Thursday March 20, 2014

Most people know not to shine lasers into sensitive places like eyeballs, but very few people can tell one laser from another. The truth is that not ALL lasers are dangerous, and there are different classifications of lasers. The most important information you need about laser safety is printed right on the laser itself.


Basic Laser Guidelines for Safety:
1. Never look directly at the beam source, or aperture
2. Never point the beam at another person
3. Always be mindful of where a “bouncing beam” will land due to reflection


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Download your student worksheet here! This download is provided by Laser Classroom. Check out their website for more free downloads and really cool lasers!


How do you enforce safety? After kids are familiar with laser classification (below), let them know that if you spot any dangerous activity around using a laser, the laser is yours (the adult) to keep forever. Period.


Are green lasers more dangerous than red lasers?


Laser Classification


Class 1 or Class I lasers do not emit hazardous levels of optical radiation. You’ll find theses types of lasers in the scanners of grocery stores at the check out counter. The beam paths and reflections are all enclosed.


Class 2 or Class II lasers are low-power visible lasers around 1 mW (milliwatt), and you’d really have to try hard to get injured by one of these types of lasers. Officially, it’s stated that this type of laser can have possible eye damage if you stare at the beam directly without blinking for at least 15 minutes.


Class 3 has two different levels of lasers, one being much more dangerous than the other.


Class 3a or IIIa lasers are 1 to 5 mW power and can’t injure you normally, but if you stare at the beam through something with lenses, like binoculars, then your eyes are toast.


Class 3b or IIIb lasers are lasers from 5mW to 500 mW, and these are the ones that cause eye injury with you look at them without any eye protection. These are NOT the ones you want kids playing with, as eye protection is always required when around these lasers.


Class 4 or IV are above 500 mW and these require not only eye protection to be around, but also skin protection. These lasers cause damage by the beam and the reflections of the beam, and are also a fire hazard.


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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:


  1. Diffraction Gratings
  2. Does Light Travel in a Straight Line?
  3. Exploring Shadows
  4. Reflecting Light
  5. Bouncing Light
  6. Adding Light
  7. Bending Light
  8. Refraction


 
Download your student worksheet here!


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Fluorescence

Tuesday September 3, 2013

Fluorescent minerals emit light when exposed to ultraviolet (UV) light, usually in a completely different color than when exposed to white light. UV is invisible to the human eye, and is the wavelength of light that is responsible for sunburns.


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Materials:


  • Longwave UV light
  • Sunlight
  • Rock samples (The four samples at the end of the video are: top left is opalite, top right is calcite, bottom left is norbergite, bottom right is calcite & willemite.)


Download worksheet and exercises


Stars, including our sun, produce all kinds of wavelengths of light, even UV. The UV minerals in this lab contain a substance that reacts with light. It takes the UV light from the sun and then re-emits it in a different wavelength that’s visible to us.


When a particle of UV light hits an atom in the mineral, it collides with an electron which makes the electron jump to a higher, more energetic state that is a bit further from the center of the atom than the electron is used to. That’s how energy gets absorbed by an atom. The amount of energy an electron has determines how far from the atom it has to be.


The electron prefers being in its lower state, so it relaxes and jumps back down, and when it does, it transfers a blip of energy away. This blip of energy is the light we see emitted from the UV mineral. This process continues as long as we see a color coming from the mineral under the UV light.


There are two different types of UV wavelengths: longwave and shortwave. Some minerals fluoresce the same color when exposed to both wavelengths, while others only fluoresce with one type, and still others fluoresce a different color depending on which it’s exposed to. Minerals fluoresce more notably with shortwave UV lamps, but these are more dangerous than longwave since they operate at a wavelength that also kills living tissue.


Shortwave UV lamps and lights should only be operated by an experienced adult. Never use a shortwave light when children are around.


Most minerals do not fluoresce, but in the ones that do, there are either small impurities that fluoresce (called “activators”) or the pure substance itself fluoresces (although this is rare). For a mineral to fluoresce, the impurities present must be in just the right amount. For example, red fluorescent calcite from Franklin, NJ, USA is activated by manganese that’s present, but only if there’s about 3% of it in the mineral. If there’s more than 5% or less than 1% manganese, the sample won’t fluoresce at all. It’s the amount and type of the impurities that determines the color and intensity of the fluorescence.


Fluorescence is not a reliable way to identify a mineral, since some samples will fluoresce with different colors even though they are all the same mineral. Fluorescence is used to determine where the mineral came from, since the colors that the minerals fluoresce usually match the original location of the mineral.


Phosphorescence is when a sample glows even after you turn off the UV light source. This is the type of glow you’ll find in “glow in the dark” toys, where the light slowly fades after you turn off the light. Atoms continue to emit light even after the electrons return to their normal energy states. While it looks like seconds to minutes that the glow lasts, some samples have been found to phosphoresce for years using highly sensitive photographic methods. Only a few minerals phosphoresce, such as calcite from Terlingua, Texas.


  1. Label and number each of your samples and record this on your data table.
  2. Hold your mineral in the sunlight and record the color in the data table.
  3. Go inside and turn off the lights. Hold your sample under a longwave UV light and record the colors that you see.
  4. Complete the data table.
    Minerals that fluoresce under longwave UV:


  • Aragonite
  • Hackmanite
  • Calcite
  • Fluorite
  • Opalite
  • Calcite & willemite
  • Tremolite
  • Resinous coal
  • Wernerite
    Minerals that fluoresce under shortwave UV:


  • Aragonite
  • Termolite
  • Wiollemite
  • Opalite
  • Chalcedony
  • Calcite & willemite
  • Talc
  • Resinous coal
  • Norbergite
  • Calcite

Exercises


  1. What wavelength is shortwave UV? Longwave UV?
  2. How is fluorescence different from phosphorescence?
  3. Name two minerals that fluoresce in both shortwave and longwave UV light.

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Measure the Track Spacing of a DVD and CD

Sunday September 1, 2013

We’re going to use a laser pointer and a protractor to measure the microscopic spacing of the data tracks on a DVD and a CD. The really cool part is that you’re going to use an interference pattern to measure the spacing of the tracks, something that you can’t normally see with your eyes.


Interference is what happens when waves smack into each other. When the waves collide, if the two highest or two lowest points of the waves are lined up, then they add together to form a larger amplitude which is seen as a bright spot of light. However, if a peak and a trough line up, then they cancel each other out and there is a dark area in the pattern (see the dark spaces in the line?).


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On the CD or DVD, when white light shines on the surface, it makes a shimmering rainbow of colors. It does this because of diffraction, which is a more complicated form of interference.


We’re going to measure the data track spacing using a diffraction pattern and a little math. Here’s how to do it:


Materials:


  • Laser pointer (red works fine in a dark room)
  • CD
  • DVD
  • Protractor
  • Index card
  • Scissors
  • Tape
  • Cardboard box
  • Stack of books
  • Marker
  • Pencil
  • Clothespin
  • Brass fastener
  • Hot glue gun
  • Piece of grid paper


The equation to determine the distance is:


dm = m λ / (sin θm – sin θi)


where  λ is the wavelength of the laser, m = the order of the refraction ray, and d is the track spacing.


Watch the video to see how to do the calculations!


Questions:


  1. Does a CD or DVD hold more data? How can you tell?
  2. Is the track spacing larger, the same, or smaller for a DVD as a CD?
  3. What is a diffraction grating?
  4. How is diffraction different or the same as interference?
  5. Why do you get more than one reflection when you shine the laser on the back surface of a CD? [/am4show]

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


  1. Make your room as dark as possible for this experiment to work.
  2. Make sure your label is removed from the glass jar or you won’t be able to see what’s going on.
  3. Fill the clear glass jar with water.
  4. Add a teaspoon or two of milk (or cornstarch) and swirl.
  5. 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.
  6. 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.
  7. 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.
  8. Cover the flashlight lens with clear tape.
  9. Paint on the tape (not the lens) the fingernail polish you need to complete the table.
  10. 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.


sunset-mars


Exercises


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

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Measure your Hair Width with a Laser

Sunday April 21, 2013

Do you have thick or thin hair? Let’s find out using a laser to measure the width of your hair and a little knowledge about diffraction properties of light. (Since were using lasers, make sure you’re not pointing a laser at anyone, any animal, or at a reflective surface.)


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Light is also called “electromagnetic radiation”, and it can move through space as a wave, which makes it possible for light to interact in surprising ways through interference and diffraction. This is especially amazing to watch when we use a concentrated beam of light, like a laser.


If we shine a flashlight on the wall, you’ll see the flashlight doesn’t light up the wall evenly. In fact, you’ll probably see lots of light with a scattering of dark spots, showing some parts of the wall more illuminated than the rest. What happens if you shine a laser on the wall? You’ll see a single dot on the wall.


In this experiment, we used a laser to discover how interference and diffraction work. We can use diffraction to accurately measure very small objects, like the spacing between tracks on a CD, the size of bacteria, and also the thickness of human hair.


Here’s what you need:


  • a strand of hair
  • laser pointer
  • tape
  • calculator
  • ruler
  • paper
  • clothespin

WARNING! The beam of laser pointers is so concentrated that it can cause real damage to your retina if you look into the beam either directly or by reflection from a shiny object. Do NOT shine them at others or yourself.



Download Student Worksheet & Exercises


  1. Tape the hair across the open end of the laser pointer (the side where the beam emits from)
  2. Measure 1 meter (3.28 feet) from the wall and put your laser right at the 1 meter mark.
  3. Clip the clothespin onto the laser so that it keeps the laser on.
  4. Where the mark shows up on the wall, tape a sheet of paper.
  5. Mark on the sheet of paper the distance between the first two black lines on either side of the center of the beam.
  6. Use your ruler to measure (in centimeters) to measure the distance between the two marks you made on the paper. Convert your number from centimeters to meters (For me, 8 cm = 0.08 meters.)
  7. Read the wavelength from your laser and write it down. It will be in “nm” for nanometers. My laser was 650 nm, which means 0.000 000 650 meters.
  8. Calculate the hair width by multiplying the laser wavelength by the distance to the wall (1 meter), and divide that number by the distance between the dark lines. Multiply your answer by 2 to get your final answer. Here’s the equation:

Hair width = [(Laser Wavelength) x (Distance to Wall)]  / [ (Distance between dark lines) x 0.5 ]


In the video:


  • wavelength was 650 nm = 0.000 000 650 meters
  • distance from the wall was 1 meter
  • the distance between the dark lines was 8 cm = 0.08 m

Using a calculator, this gives a hair width of 0.000 0162 5meters, or 16.25 micrometers (or 0.000 629 921 26 inches). Now you try!


What’s Going On?


The image here shows how two different waves of light interact with each other. When a single light wave hits a wall, it shows up as a bright spot (you wouldn’t see a “wave”, because we’re talking about light).


When both waves hit the wall, if they are “in phase”, they add together (called constructive interference), and you see an even brighter spot on the wall.


If the waves are “out of phase”, then they subtract from each other (called “destructive interference”) and you’d see a dark spot. In advanced labs, like in college, you’ll learn how to create a phase shift between two waves by adding extra travel length to one of the waves along its path.


So why are there dark lines along the light line when you shine your laser on the hair in this experiment? It has to do with something called “interference”.


One kind of interference happens when light goes through a small and narrow opening, called a slit. When light travels through a single slit, it can interfere with itself. This is called diffraction.


When light travels through one of two slits, it can interfere with light traveling through the other slit, a lot like how water ripples can interfere with each other as they travel over the surface of water.


If you’re wondering where the slit is in this experiment, you’re right! There’s no narrow opening that light it traveling through. in fact, light appears to be traveling around something, doesn’t it? Light from the laser must travel around the hair to get to the wall. The way that light does this has to do with Babinet’s Principle, which relates the opposite of a slit (a small object the size of a slit) to the slit itself.


It turns out amazingly enough that when light hits a small solid object, like a piece of hair, it creates the same interference pattern as if the hair were replaced with a hole of the same size. This idea is called Babinet’s Principle.


By measuring the diffraction pattern on the wall, we can measure the width of a small object that the light had to travel around by measuring the dark lanes in the spot on the wall. In our lab, the small object is a piece of your hair!


Questions to Ask:


  1. What would happen to the diffraction pattern if the hair width was smaller?
  2. Using this experiment, how can you tell if the hair is round or oval?
  3. If we redid these experiments with a different color laser instead of red, what changes would you have needed to make?
  4. How can you modify this experiment to measure the width of a track on a CD? Does the track width change as a function of location on the CD? If so, is it larger or smaller near the outside?

Exercises 


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

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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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‘Lasers’ 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. What does LASER stand for?


2. How is a laser different from an incandescent bulb?


3. What are two things that can split a laser beam?


4. How do you make a laser beam visible?


5. What’s the secret behind the laser light show?


6. How do lasers damage things?


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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Answers to ‘Lasers’ 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.


1. Light can change speed The word “LASER” stands for Light Amplification by Stimulated Emission of Radiation.


2. Light from a regular incandescent light bulb covers the entire spectrum as well as scatters all over the room. A laser beam is monochromatic – the light that shoots out is usually one wavelength and color, and is in a narrow beam.


3. Glass (like a window pane) and clear plastic (like a water bottle).


4. Take it in a steamy room, like just after a hot shower. Or aim it through a glass of water that has a drop of milk in it.


5. The laser beam hits a spinning mirror that’s off-center. The more angled the mirror mount, the larger the image that the laser traces out. Which is why this is a perfect project for kids – the sloppier they build it, the better the laser light show.


6. High-power CO2 lasers have an intense amount of heat that melts through metal. These aren’t the lasers we’re going to be working with! The lasers at the grocery store are Class I lasers, which will harm your eye if you stare into it without blinking once for at least 15 minutes. These ‘keychain’ lasers are Class II & III, some of which can overpower your retina in less than a minute, and the damage is irreversible. When I work with kids in a live Laser Lab class, I have a zero-tolerance rule (which is explained beforehand): if misused, I just walk over, take the laser without a word, and keep it. Class proceeds as normal, and it’s up to the kid to figure out how to finish the project.


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 (90o).  The lenses should block the light completely at 90o and allow light to pass-through when aligned at 0o. These lenses allow some light to pass through but not all. When you rotate the lenses to 90o, 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 90o 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


  1. Why do you need two polarizers to block the light completely?
  2.  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


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

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