A container of rarefied gas whose atoms are practically all in the ground state is illuminated by a

Question

A container of rarefied gas whose atoms are practically all in the ground state is illuminated by a continuous monochromatic light source. The gas strongly absorbs the light, leading to resonance excitation. The wavelength of the resonant line is $\lambda = 248 	ext{ nm}$. As a result of this illumination, the gas begins to emit radiation at this same wavelength, achieving a steady-state emitted radiation power of $P = 50 	ext{ mW}$. The mean lifetime of the atoms in this state of resonance excitation is $	au = 0.16 \mu s$.
(Take Planck's constant $hc = 1240 	ext{ eV nm}$ and $1 	ext{ eV} = 1.6 	imes 10^{-19} 	ext{ J}$)

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