How do lasers work?

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.

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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 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.

Source: education.jlab.org/qa/laser_01.html