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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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]

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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Laser Basics

Wednesday February 3, 2010

Did you know that the word LASER stands for Light Amplification by Stimulated Emission of Radiation? And that a MASER is a laser beam with wavelengths in the microwave part of the spectrum? Most lasers fire a monochromatic (one color) narrow, focused beam of light, but more complex lasers emit a broad range of wavelengths at the same time.


In 1917, Einstein figured out the basic principles for the LASER and MASER by building on Max Planck’s work on light. It wasn’t until 1960, though when the first laser actually emitted light at Hughes Research Lab. Today, there are several different kinds of lasers, including gas lasers, chemical lasers, semiconductor lasers, and solid state lasers. One of the most powerful lasers ever conceived are gamma ray lasers (which can replace hundreds of lasers with only one) and the space-based x-ray lasers (which use the energy from a nuclear explosion) – neither of these have been built yet!


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Gas lasers pump different types of gases to get different laser colors such as the red HeNe (Helium-Neon laser), the high-powered CO2 lasers that they can melt through metal, the blue-green argon-ion, the UV lasers that use nitrogen, and the metallic-gas combination such as He-Ag lasers (helium and silver) and Ne-Cu (neon and copper) which emit a deep violet beam.


But what about lasers used everyday? The lasers we’re going to be using are semiconductor lasers that use a small laser diode to emit a beam. They are the same lasers that are in the grocery store scanners, pen laser pointers and key chain lasers. Usually a class I or II laser, these pose minimal safety risk and are safe to use in our experiments. Here’s what you need to know:


Materials:


  • laser (A key-chain laser works great. Do NOT use green lasers, which can only be used outdoors.)
  • dark room
  • old CD
  • cut-crystal (wine glasses, fancy vases, etc) with adult help
  • microscope slide or window
  • cellophane and nail polish (red, green, and blue are optimal and used again in the Light Wave experiments)
  • feather
  • two pairs of polarized sunglasses
  • frosted incandescent light bulb


Before we start building our laser projects, just play with it first. Turn off the lights at night and take your laser on a hunt around the house to see what happens when you shine it on or through different things. Here are some ideas to try:


1. Shatter the Beam: Shine your beam over the surface of an old CD. Does it work better with a scratched or smoother surface? You should see between 5-13 reflections off the surface of the CD, depending on where you shine it and how well the “seeing” conditions are.


2. Beam Scatter: Pass the laser beam through several cut-crystal objects such as wine glasses or clear glass vases. Is there a difference between clear plastic or glass, smooth or multi-faceted? Try an ice cube, both frosted and wet (clear).


3. Split the Beam: Shine the laser beam through a flat piece of glass, such as a window. Can you find the pass-through beam as well as a reflected beam? Windows and clear plastic containers will split your beam in two.


What’s going on? When you shine your laser beam through glass (like a window) or plastic (like a soda bottle filled with water and a tiny bit of cornstarch), it splits into two beams – one that passes through, and the other that internally reflects back. You can see these reflections in a darkened fog-filled room.


4. Colored Filters: Paint a piece of cellophane or stiff clear plastic with nail polish (or use colored filters) to put in the laser beam.


5. Diffraction Grating: You can make a quick diffraction grating by using a feather in the beam.


6. Polarization: If you have polarizer filters, use two. You can substitute two pairs of sunglasses. Just make sure they are polarized lenses (most UV sunglasses are). Place both lenses in the beam and rotate one 90 degrees. The lenses should block the light completely in one configuration and allow it to pass-through the other way.


Why does this happen? Polarization is a way of filtering light. Try this: in a shallow pan filled with water, make a few waves and notice how they travel from one side of the pan to the other. Now add a plastic comb, and notice how the waves stop when they hit the comb – not many pass through to the other side (watch out for the waves that creep around the edges – we’re focusing on the pass-through waves only). The comb are the sunglasses, and the water waves are the light waves.


Add a second comb at 90 degrees from the first (as you did with the sunglasses) so it resembles a mesh screen, and notice now how NONE of the waves make it through the comb array. Polarization can filter out various amounts of light, depending on the angle the combs make with each other (90 degrees apart equals total block-out).


7. Light Bulbs: In the dark, aim your laser at a frosted incandescent light bulb. The bulb will glow and have several internal reflections! What other types of light bulbs work well?


Learn more! Read up about lasers here.


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Laser Maze

Wednesday February 3, 2010

By using lenses and mirrors, you can bounce, shift, reflect, shatter, and split a laser beam. Since the laser beam is so narrow and focused, you’ll be able to see several reflections before it fades away from scatter. Make sure you complete the Laser Basics experiment first before working with this experiment.


You’ll need to make your beam visible for this experiment to really work.  There are several different ways you can do this:


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1. Take your laser with you into a steamy bathroom (which has mirrors!) after a hot shower.  The tiny droplets of water in the steam will illuminate your beam. (Psst! Don’t get the laser wet!)


2. If you have carpet, shine your laser under the bed while stomping the floor with your hand.  The small particles (dust bunnies?) float up so you can see the beam. Some parents aren’t going to like this idea, sooo….


3. Drop a chunk of dry ice (use gloves!) into a bowl of water and use the fog to illuminate the beam.  The drawback to this is that you need to keep adding more dry ice as it sublimates (goes from solid to gas) and replacing the water (when it gets too cold to produce fog).


Materials:


  • large paper clips
  • brass fastener
  • index card
  • small mirrors (mosaic-type work well)


Download Student Worksheet & Exercises


Here’s what you do: Open up each paper clip into the “L” shape.  Insert a brass fastener into one U-shape leg and punch it through the card.  Hot glue (or tape) one square mirror to the other end of the L-bracket.  Your mirror should be upright and able to rotate.  Do this with each mirror.  (You can alternatively mount each mirror to a one-inch wooden cube as shown in the video.)


Turn on the laser adjust the mirrors to aim the beam onto the next mirror, and the next!  Turn down the lights first and use any one of the methods mentioned above to make your laser beam visible.


What’s happening? The mirrors are bouncing the laser beam to each other, and the effect shows up when you dim the lights and add fog or dust particles to help illuminate the beam.  A laser beam is a highly focused beam of light, and you can direct that light and bounce it off mirrors!


Why can’t I see the beam normally? The reason you can’t see the laser beam without the help of a steamy room, dirty carpet, or fog machine is that your eyes are tuned for green light, not red (which is why you can see the beam from a green laser at night).


Exercises


  1.   The word LASER is actually an acronym. What does it stand for?
  2. What type of laser did we use in our experiment?
  3. Why can’t we see the laser beams without the help of steam, dirty carpet, etc.?

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Laser Light Show

Wednesday February 3, 2010

ss-laserWhat happens when you shine a laser beam onto a spinning mirror? In the Laser Maze experiment, the mirrors stayed put. What happens if you took one of those mirrors and moved it really fast?


It turns out that a slightly off-set spinning mirror will make the laser dot on the wall spin in a circle.  Or ellipse. Or oval.  And the more mirrors you add, the more spiro-graph-looking your projected laser dot gets.


Why does it work? This experiment works because of imperfections: the mirrors are mounted off-center, the motors wobble, the shafts do not spin true, and a hundred other reasons why our mechanics and optics are not dead-on straight.   And that’s exactly what we want – the wobbling mirrors and shaky motors make the pretty pictures on the wall!  If everything were absolutely perfectly aligned, all you would see is a dot.


Here’s how to do this experiment:


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Materials:


  • AA battery pack with AA batteries
  • two 1.5-3V DC motors OR rip them off old toys or personal fans sold in the summertime
  • keychain laser pointer
  • clothespin
  • two round mirrors (mosaic mirrors from the craft store work great)
  • two alligator clip leads
  • gear that fits onto the motor and has a flat side to attach to the mirror
  • 5-minute epoxy (don’t use hot glue – it’s not strong enough to hold the mirrors on at high motor speeds)

**Note – if this is your very first time wiring up an electrical circuit, I highly recommend doing this Easy Laser Light Show first. It uses a lot of the same parts, but it’s easier to build.**



Here’s what you do:


1. Insert the batteries into their case.


2. Use 5-minute epoxy to secure the gear onto the round mirror.


3. Press-fit the gear-mirror onto the shaft when the epoxy is dry.


4. Make the motor spin using the alligator clips and the battery case.


5. Turn down the lights and fire up the laser, aiming the beam onto the motor.


6. Shine the reflection somewhere easy to see, like the ceiling.


7. Once you’ve got this working, add a second mirror like you did in the laser maze.


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Advanced Two-Axis Laser Light Show

The second part of this experiment is for advanced students. What shapes can you make?  Is it tough to hold it all in place? Then here’s how to create a portable laser light show:


Materials:


  • Red laser pointer
  • 3VDC motors
  • 2 gears or corks (you’ll need a solid way to attach the mirror to the motor shaft tip)
  • two 1” round mirrors (use mosaic mirrors)
  • 2 DPDT switches with center off
  • 20 alligator clip leads  OR insulated wire if you already know how to solder
  • 2 AA battery packs with 4 AA’s
  • Two 1K potentiometers
  • Zip-tie (from the hardware store)
  • ½ ” or ¾ ” metal conduit hangers (size to fit your motors from the hardware store)
  • 3 sets of ¼ ” x 2″ bolts, nuts, and washers
  • 1 project container (at least 7” x 5”) with lid
  • Basic tools (scissors, hot glue gun, drill, wire strippers, pliers, screwdriver)

Click here to download the Schematic Wiring Diagram for the Advanced Laser Light Show.



Download Student Worksheet & Exercises


Exercises


  1. How does the mirror turn a laser dot into an image?
  2.  What happens when you add a second motor? Third?

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Space-Age Laser Communicator

Wednesday February 3, 2010

laser17This experiment is for advanced students.Did you know that when you talk inside a house, the windows vibrate very slightly from your voice? If you stand outside the house and aim a laser beam at the window, you can pick up the vibrations in the window and actually hear the conversation inside the house.


Remember how windows split a laser beam in two from the Laser Basics experiment? That’s the basic idea behind it. First, I’ll show you how to build your own space-age laser communicator, then you can work on your spy device.


The first thing we’re going to do is take the music from your stereo or MP3 player and transmit it on a laser beam to a detector on the other side. The detector has an earphone attached, so someone else can listen to the music from your laser. Weird, huh?


Here’s how to build your own:


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Materials:



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