Problem 6.128

Rarefied Hg gas whose atoms are practically all in the ground state was lighted by a mercury lamp emitting a resonance line of wavelength $\lambda=253.65 \mathrm{nm}$. As a result, the radiation power of Hg gas at that wavelength turned out to be $P=35 \mathrm{~mW}$. Find the number of atoms in the state of resonance excitation whose mean lifetime is $\tau=0.15$ us.