Laser Physics?

Question

An atom has two inergy levels with a transition wavelength of 694.3 nm. Calculate the % of atoms in the upper level at T=300 K

Answer 1

"The number of these atoms which are in the ground state is given by N1, and the number in the excited state N2. Since there are N atoms in total,

N1 + N2 = N

The energy difference between the two states is given by

ΔE = E2 − E1

determines the characteristic frequency ν21 of light which will interact with the atoms; This is given by the relation

E2 − E1 = ΔE = hν21,

h being Planck's constant.

If the group of atoms is in thermal equilibrium, it can be shown from thermodynamics that the ratio of the number of atoms in each state is given by a Boltzmann distribution:

{N2}/{N1} = exp{-(E2-E1)/(kT)},

where T is the temperature of the group of atoms, and k is Boltzmann's constant. "
All of this from
http://en.wikipedia.org/wiki/Population_inversion

Answer 2

A two-way mirror, or a half-silvered mirror, same thing, lets through about half the light that strikes it. So most of the light you fired at the two-way mirror would come straight back at you. The only losses would be the tiny amount of light absorbed by the ordinary mirror. Most lasers have an ordinary mirror at one end and a half-silvered mirror at the other end of the lasing material. The laser beam emerges from the half-silvered mirror. If you're planning to do this experiment, remember that even a weak laser like 10 mW can damage your eyes because, even though that's about as much light as a torch bulb, it's concentrated in a narrow beam, so wear dark glasses and don't use a powerful beam.