Consider a hydrogen atom undergoing a 2p-1s transition. The frequency (v) corresponding to this transi- tion is 2.47 x 105 Hz and the life-time of 2p state for spontaneous emission is 1.6 ns. Assuming that the refractive index (no) of the medium is 1, the co-efficient of stimulated emission [B, (v)] for this transition is given by 0.109 x 10 m W- sec- 0.68 x 10 m W sec- O 0.109 x 1021 m W- sec- O 6.8 x 109 m W- sec-