(06-07-2011 02:06 PM)BlazerFan11 Wrote: Thank you for copying and pasting from Wikipedia what the acronym stands for. I never could have done that on my own.
Ok, let's see if I can do this faster than finding a good site in the interweb.
Atoms/molecules can absorb energy or emit energy. One form of energy is light, and when atoms absorb/emit light, it is in a quantized unit (discrete packet of energy) called a photon.
All atoms can absorb light, they can take in more energy. Not all atoms can emit light, they don't have extra energy, these are ground-state atoms. Atoms that can emit light are said to be in an excited state (ugh, this is gonna go downhill fast w/ you yahoos)
Now, when an atom absorbs light, obviously a photon has to be present...it gets absorbed.
When an atom emits light, it doesn't need another photon around, just the one it emits. If it does this, it's called spontaneous emission. And as the name indicates, this just happens; it's a probability event for each emission, which means a certain number emit per unit time.
But...an excited state atom can have another photon around, and in the reverse of absorption (anti-absorption) this photon can cause "stimulated emission". Then you have two photons, and these photons are "in phase" (meaning their wave properties match up) leading to high intensity light and they travel in the same direction, so the light is already pretty tightly focused.
Of course, when you get all (or actually more than half) the atoms to emit photons you run out of enough excited state atoms for stimulated emission, so your process stops. To keep it up, lasers take a lot of energy generating excited state atoms. Thus they're badly inefficient b/c so much energy is lost to spontaneous emission by the atoms.
So you have this process going on in a laser, and most designs have the light bouncing back and forth between two mirrors, through the container of excited state atoms, thus the laser intensity grows. But one of the mirrors usually allows some light out, so you have your laser beam.