The word "LASER" stands for Light Amplification by Stimulated Emission of Radiation. A laser is an optical light source that emits a concentrated beam of photons. Lasers are usually monochromatic – the light that shoots out is typically one wavelength and color, and is in a narrow beam.
By contrast, light from a regular incandescent light bulb covers the entire spectrum as well as scatters all over the room. (Which is good, because you could light up a room with a narrow beam of light?)
There are about a hundred different types of atoms in the entire universe, and they are always vibrating, moving, and rotating. Think of kids on sugar. When you add energy to these atoms (even more sugar to the kids), they really get excited and bounce all over the place.
When the atoms relax back down tot heir "normal" state, they emit a photon (a light particle). Think of the kids as coming down from their sugar high, and they all collapse on the couch.
A laser controls the way energized atoms release photons. Imagine giving half the kids sugar, and picture how they would bounce all over the place (like light from a bulb) when it took effect. They would be very high-energy among the other half who were potentially sitting down.
Now imagine those sugar kids jumping in unison (a focused laser beam). The sugar-kids are infectious, and pretty soon, the kids around them are joining in and sharing in their excited energy. This is how a laser charges the atoms inside the gas medium.
Now imagine a cat-flap that lets out a limited number of kids out at a time, while the rest are bouncing around inside, charging up everyone. That cat-flap exit is the laser beam exiting the laser. The atoms remaining inside the laser bounce off mirrors as they charge each other up.
Before we start, you'll need eye protection – tinted UV ski goggles are great to use, as are large-framed sunglasses, but understand that these methods of eye protection will not protect your eyes from a direct beam. They are intended as a general safety precaution against laser beam scatter and spinning mirrors. (Yes, you will be wearing sunglasses in the dark!)
A very neat addition to the experiments below is a fog machine. (Rent one from your local party supply store.) Turn it on, be sure you have good ventilation, darken the lights, and turn on the lasers for an outstanding laser experience!
A quick note about lasers: keychain lasers from the dollar store work just fine with these projects. Do not use the green lasers sold in astronomy stores – they are too dangerous for the eyes.
Plastic Bottle Beam: Fill up a plastic water or clean soda bottle with water and add a sprinkle of cornstarch. Turn down the lights and turn up the laser, aiming the beam through the bottle. Do you see the original beam in the bottle? Can you find the reflection beam and the pass-through beam?
Light Bulb Laser: 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?
CDs: Shine your beam over the surface of an old CD or DVD. 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 good your "seeing" conditions are.
Glass and Crystal: 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.
Lenses: If you have an old pair of eyeglasses, pop out the lenses and try one or both in the beam to see the various effects. Try one lens, and then try two in line with each other to see if you can change the beam. If you have polarizer filters, use two. You can substitute two sunglass lenses – no need to pop out the lenses – you can just use 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.