Energy can take one of two forms: matter and light (called electromagnetic radiation). Light is energy that can travel through space. When you feel the warmth of the sun on your arm, that’s energy from the sun that traveled through space as infrared radiation (heat). When you see a tree or a bird, that’s light from the sun that traveled as visible light (red, orange… the whole rainbow) reflecting and bouncing off objects to get to your eye. Light can travel through objects sometimes… like the glass in a window.

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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?

Click here to go to next lesson on Light Reflection

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You can see objects because light from that object travels to your eyes. Sometimes light is reflected off objects before it reaches our eyes, and sometimes it comes straight from the source itself.

A candle is a light source. So is a campfire, a light bulb, and the sun. An apple, however, reflects light. It doesn’t give off any light on its own but you can see it because light waves bounce off the apple into your eye. If you shut off the light, then you can’t see the apple. In this same way, the sun is a light source, and the moon is a light reflector.
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You see objects because light reflections because light reflects off them into your eye. The angle that the light wave approaches it equal to the angle that the wave leaves the surface, which is not only true for light but also sound waves.

Click here to go to next lesson on Does Light Travel in a Straight Line?

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Does light travel in a straight line? Let’s find out…

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Click here to go to next lesson on Light Refraction

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Light also bends as it passes from one medium to another, like going from the air to a glass window. But why does that happen?
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You can imagine a toy car going from a wood floor to carpeting. One wheel hits the carpet first and slows down before the other, causing the toy to turn. The direction of the wave changes in addition to the speed. The slower speed must also shorten its wavelength since the frequency of the wave doesn’t change.

The bottom line is that bending is caused by the change in speed of light when it crosses a boundary. This is true everywhere, even in the vacuum of space if it’s going from space to our atmosphere.

Click here to go to next lesson on Light Refraction using Two Lasers

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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?

Click here to go to next lesson on Diffraction

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Diffraction happens when light goes around obstacles in its path. Sound waves diffract bend around obstacles, so if you’re stuck behind a pillar at a concert, you can still hear just fine.

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

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

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

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

Materials:

• feather
• old CD or DVD

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

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

Materials:

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

Download Student Worksheet & Excercises

Exercises

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?

Click here to go to next lesson on Useful Diffraction

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?

Click here to go to next lesson on Introduction to Lasers

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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:

• flashlight
• laser
• diffraction grating or old CD
• clear tape
• red, green, and/or blue fingernail polish

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

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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Click here to go to next lesson on Light Interference

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.

Click here to go to next lesson on More on Light Interference

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Let’s take a closer look at the interference patterns of light:

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Click here to go to next lesson on Thin Film Interference

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Ever noticed a rainbow on top of a water puddle where there is oil floating on top?

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The oil is spread in a very thin layer on top of the water, and when light hits this thin layer, it interferes and causes a rainbow to be seen.

Click here to go to next lesson on Polarization

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

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

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

Materials:

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

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

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

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

Download Student Worksheet & Exercises

Advanced students: Download your Polarization lab here.

Exercises

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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Click here to go to next lesson on Shadows

Here’s a neat experiment you can do with shadows…

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Click here to go to next lesson on Color and Vision

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