05/14/10 PHD comic: 'Tales from the Laser Beam'

Greg Bernhardt

Piled Higher & Deeper by Jorge Cham

More...

BlackGriffen

I'm sure I'll be quickly corrected by someone who actually knows what they're talking about, but here goes nothing. The atom doesn't emit two photons in a laser. What does happen is the presence of a photon near the atom makes it more likely to emit a photon and for the emitted photon to have similar properties (polarization, momentum, phase) as the stimulator. This is because photons are bosons, and always exist in a state that is symmetric under exchange of particles. It may not be obvious why one thing has anything to do with another, but that symmetry means that photons "like to" clump together in the same state, just like the two Fermions are unable to have the same state.

So the hit atom doesn't emit two photons, in fact I'm not sure it's accurate to describe the atom as "hit" since the stimulating photon doesn't deviate and the atom doesn't jump energy levels, as far as I know. What is happening is that there's an atom that's ready to release a new child photon on the world and the presence of another photon with the same energy makes the new photon, susceptible to peer pressure as photons are, more likely to run off and join the crowd. :)

vocaro

Typo: "and drops a levels"