Laser Basics

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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