Given that B12N1ρ(υ12) = A21N2 + B21N2ρ(υ12) show that for a very large photon density ρ(ν12) → ∞ it follows that B12 = B21. If the absorption of a photon causes an electron to make a direct...

Given that

B12N1ρ(υ12) = A21N2 + B21N2ρ(υ12)

show that for a very large photon density ρ(ν12) → ∞ it follows that

B12 = B21.

If the absorption of a photon causes an electron to make a direct transition

(i.e., no phonon involved) from a state with wavevector |k| = 1.00 × 107

cm−1 in a given direction to a state with wavevector |k| = 1.01 × 107 cm−1

in the same direction, what was the wavelength of the photon?