A show, often performed in planetariums, where colored lasers are used to make interesting patterns on a screen/ceiling/whatever. Usually combined with loud music; I've seen laser shows using many different genres of music, but 60s/70s classic rock seems to be the most popular.

Let's talk about why laser light isn't like oter light sources:

Laser light has several very unique properties that separate it from other forms of light. These properties are what make lasers useful in labs, in cd players, and in laser light shows.

First, laser light is monochromatic. The electro-magnetic spectrum encompasses hundreds of different forms of energy. All of these different types of energy are classified by their wavelength. The numbers on a radio dial, for instance, are not arbitrary but correspond to the wavelength of radio signal on which a particular station broadcasts. Other portions of the electro-magnetic spectrum include everything from gamma rays and x-rays to microwaves and cellular phone signals. What we call light is actually just a portion of this spectrum which contains wavelengths that our eyes can intercept. Each distinct wavelength of light is perceived as an individual color. Incandescent light sources; anything from desk lamps to a theatrical fresnel all produce a broad spectrum of light. That is, they contain many colors. The addition of a gel to the front of a lighting instrument filters all but the specific wavelengths necessary to produce that gel color.

Lasers do not produce this broad spectrum of light and instead only produce very specific wavelengths (colors) of light. Most produce just one color, one specific wavelength. Others combine special gases to produce a few wavelengths of red, blue, and green, which are combined to produce many different colors. In fact, many laser artists, called laserists, refer to colors not by their name, but rather by their specific wavelength, such as 532 nanometers, the wavelength of a brilliant green produced by some YAG lasers.

In addition to being monochromatic, laser light is directional. Unlike other light sources, where the light escapes in many different directions, laser light only travels along a single line or plane. In basic terms, this means that laser light goes where it is directed. Provided your optics are clean, nowhere else. This makes lasers a very useful measuring and sampling device. The photons, or tiny packets of light that compromise the laser beam all travel together in exactly the same direction.

Another important aspect of laser light, tied to being directional, is laser light’s low divergence. Divergence is a measurement of how much a light spreads over a given distance. Laser light does not expand noticeably over short distances. Even after traveling thousands of feet a laser beam may only be a few inches wide. Compare this to a small flashlight. Even with lenses to focus the light, the source goes from being 2 centimeters wide at the lense to as much 60 centimeters wide over just a few feet.

Laser light is also coherent. That is, all of the photons in the laser beam travel in perfect phase with each other. You could say that the photons are all in step. It is actually this coherence that makes laser light have a low divergence. Coherence is a property that is especially usefull to scientists, again because unlike other light sources, this is a very predictable, repeatable property.

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