Problem 6.127

Calculate the mean lifetime of excited atoms if it is known that the intensity of the spectral line appearing due to transition to the ground state diminishes by a factor \(\eta=25\) over. a distance \(l=2.5 \mathrm{~mm}\) along the stream of atoms whose velocity is \(v=\) \(=600 \mathrm{~m} / \mathrm{s}\)