Question

What will be the relative population of atoms in a ruby layer that produces a light beam of wavelength 6943 Å at 300 K.

(a) 5 X 10^-31

(b) 6 X 10^-31

(c) 7 X 10^-31

(d) 8 X 10^-31

The question was asked in an interview for internship.

My question is based upon Population Inversion topic in section Laser of Engineering Physics II

Answer 1

The correct answer is (d) 8 X 10^-31

Explanation: The relative population of atoms in two states with energy E1 and E2 is given by

N2/N1 = e\(\frac{-(E_2-E_1)}{kT}\)

E2 – E1 = hv

= hc/λ

= 1.79 eV

Therefore, N2/N1 = \(e\frac{-1.79}{kT} \)

= e^-69.3

= 8 X 10^-31.