Well, I know that if light of the right color hits an atom, it will bump an electron up to
a higher energy level. And later the electron falls back down, giving off light of the same
color in some random direction.
Right. But Einstein, more than 60 years ago, realized that there was also something else
that could happen. See if you can discover this by playing with the applet below. Adjust the
"brightness" of the light source, which makes the particles of light called photons, come
out faster and see if you notice anything curious happening:
Something weird is going on...when a photon hits an atom that is already excited, it looks
like the atom lets go of the photon.
Did you notice in what direction the photon goes?
Let's see.... Yes, it ends up going in the same direction as the incoming photon.
Yes! This is the key thing that Einstein figured out that makes lasers possible. When a
photon hits an atom that is already excited, the atom releases a new photon that is
completely identical to the incoming photon; same color, going in the same direction.
We call this process "stimulated emission".
I think I get it...one photon hits an excited atom and then we have two photons travelling
together. When one of those finds another excited atom we get three photons, and so on and
so on, but they are all exactly the same because they are being cloned by stimulated emission.
Exactly, so the number of photons gets amplified. The word laser is short for "light
amplification by stimulated emission of radiation".
I can see why they shortened it!
By the way, we're representing the light as little particles in this picture, but you can
also think of it as waves. Then the incoming light is a wave, and when it hits the excited
atom, the atom releases some energy that just makes the wave get bigger.