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.