Common Questions

What about laser weapons?

Somebody always asks a question about laser weapons. In reply, I always emphasize that lasers don't blow things up, like in Star Wars; rather, the effect is more like focusing the light from the sun with a magnifying glass onto a piece of paper to burn holes in it (most of the students I presented to had experience with this). It is just a lot of energy concentrated on one spot, which heats up whatever it's shining on. This makes lasers an ineffective weapon to use against a person, for a person will usually move out of the way as soon as they feel a burning. Also, the damage inflicted by a laser is generally confined to the surface of the body, and so is fairly minor (compared to that inflicted by, say, a bullet). Another consideration is that lasers are very inefficient, i.e. most of the power going into the laser doesn't emerge in the beam. Much of it, in fact, is dissipated as heat, which means that a laser powerful enough to cause damage to a person (or machine) would burn the hand of the person holding it! Some lasers, mounted in tanks, are used directly as weapons, but they aren't terribly common; lasers in warfare are usually used for targeting and rangefinding.

Why does the spot look speckled?

This is because the light coming out of the laser is coherent, that is, all the light waves are in phase. When this coherent light strikes a rough surface, such as paper or the wall, it scatters and creates an interference pattern, which is the speckle. Speckle is one of the distinctive properties of laser light that probably most people who have seen lasers can recognize, though they may not be able to describe exactly what it is that's different about it.

What are lasers used for?

When the laser was first invented, some wit labeled it 'a solution looking for a problem,' because it was such a wondrous-seeming device that it had to be useful, but nobody (except a few research physicists) could think of what to do with it. That state of affairs is no longer the case, and lasers are used in many places in many different fields. Applications students would probably be most familiar with are CD players and related video-disc players and CD-ROMs, and the scanners in supermarkets and other stores that read the bar codes on products. One application that is growing more and more common is in communications: lasers and fiber optics can transmit much more information than a conventional phone line, and are gradually replacing most of them. Lasers are also used in surgery, as they can be focussed to a small enough point to vaporize individual cells without harming surrounding tissue. Because they always travel in a straight line, lasers are useful in surveying and construction; they can also measure distances to very high accuracy. Powerful lasers are used in industry for welding and cutting, and in the military for targeting and as weapons themselves. Holograms need to be produced (and often viewed) with lasers. Lasers are also used extensively in many areas of scientific research.

How does a CD player work?

The surface of a CD is made of a reflective material with many tiny pits marked in it, in a long continuous spiral like that on a vinyl record. The CD player shines a laser along this groove, and records whether it is reflected (from the untouched surface) or scattered (by a pit). This data is converted into a string of ones and zeroes, which is then converted into a sound signal by a small computer inside the player.

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