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How do lasers work?

Wow, great question... but to answer it completely I would have to take you all the way through college physics. Let me try anyway, but first let's define what a laser and laser light are.

Laser is an acronym for Light Amplification by Stimulated Emission of Radiation. The next thing to consider is why laser light is different than other light. You know that laser light travels in straight lines and stays in a small beam instead of spreading out like regular light, right? The trick there is that laser light is only one color. You've seen rainbows I am sure. What we call white light (like sunlight) is actually a mixture of a whole range of colors from blue to red. All of those colors travel together all mixed up. When the colors hit something that makes them bend, like raindrops, prisms or almost anything for that matter, those colors separate.

Imagine a race track jammed full of cars all going different speeds. They travel in a bunch until they come to a curve. The cars going faster cannot turn sharply so they go to the outside of the track. The slower cars can turn sharply so they move to the inside of the track. When the cars come out of the curve the cars are arranged from fastest to slowest. The same thing happens with light although it's the lights energy level or color that separates them. Now imagine that every single car is going exactly the same speed. The cars go into the curve in a line, go through the curve and come out of the curve still in a straight line. That's how laser light works. It does bend when it hits something, but all the light gets bent the same amount, so the light does not spread out.

So again, a laser generates a light that is rigorously one color. How that is done is both extremely simple and very complex at the same time. A property of electrons (check out Jefferson Lab's Tour of the Atom) is that, after being excited or energized to a higher than normal state, they will eventually fall back to their original state. The energy that they had at that higher level leaks away as light of a specific color. If we excite a lot of electrons they leak off a lot of light all of one color. We do this a number of ways. A very simple way is to take material that has the right electrons and flash a strong light on it. The electrons in that material will absorb the energy and spit it back out as a single color light. We use devices like mirrors and lenses to get all of the light traveling in the same direction and off it goes in straight line. Since laser light does not scatter very much you usually cannot see it until it hits something.

Here at Jefferson Lab we are building one of the coolest lasers in the world. Usually it takes a lot of energy to raise the energy of electrons and is therefore expensive to do. The electrons we use here to study the nucleus of the atom are energized relatively economically because we use a superconducting accelerator (we'll save that for another discussion). The electrons we use can be used as a source of laser light just by wiggling them as they fly through a tube. Wiggling the electrons makes them give off some of their energy as light that again is all the same color, a color that we can select. There are all sorts of uses for this very powerful laser, for instance it can be used to make very hard steel very smooth.

There are lasers in use all around us, probably several in your house. There are lasers in your CD player and the CD ROM in your computer. Lasers are used in smoke detectors. Very powerful lasers are used in machine shops to cut and weld metal. Because laser light travels straight, lasers are used for many types of alignment and measurement. Construction workers now routinely use lasers to level floors and to insure a stripe painted on a wall is straight. NASA left a mirror on the moon during one of the Apollo missions, which they bounce a laser off of to measure very precisely how far away from the earth the moon is.


Brian Kross, Chief Detector Engineer (Other answers by Brian Kross)

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