What is a "laser"?

What do you think of when you hear about "lasers"? Do you think of science fiction movies? eye surgery? CD players? These are some of the ways in which lasers are used but there are many others, as well. They are often used in scientific research as a tool for studying other materials and for information retrieval and recording. Scientists have also found a use for them in time-keeping.

A laser can be thought of as simply a light source that emits only one wavelength of light. Unlike a standard light bulb or sunlight, a laser emits only one wavelength, one tiny portion of the spectrum. To learn more about the spectrum of light go to Submodule 3.

Not only do lasers emit only one wavelength, the light waves that make up the laser beam tend to line up , trough-to-trough and crest-to-crest. In scientific terms, this is called coherence.

These are the two special properties of lasers: monochromatic (one-wavelength) emission and coherence. Because of these special properties, lasers can tell us about things when the laser light is shined on an object and then interfered with itself. This interference is a lot like colliding waves in a pool or ocean. When two waves collide, if the peaks meet, then the waves will add together and you'll see a bigger wave. If a peak and a trough collide, then the waves will subtract each other and you'll see a flat spot.

Move around. Does the speckle pattern move, too? If you're near-sighted or far-sighted and you wear glasses, take them off and view the spot. Can you still see it? If you move around with your eyeglasses off, which way does the speckle pattern move? Compare your observations with others who have eyeglasses.