When an atom transits from an excited state to ground state in the presence of an external radiation then it is called as stimulated transition.When an atom transits from an excited state to ground state on its own then it is called as spontaneous transition.Ratio of the coefficient for stimulated transition to spontaneous transition is given by:R=Bρ(ν)A​=ekThν​−1 For microwave region​ν=1010( say )R=e1.38×10−23×3006.6×10−34×1010​−1=e0.0016−1=0.0016​ This implies that stimulated transition dominate in this region.For visible region​ν=1015⇒R=e160−1⇒R>>1​ So here spontaneous transition dominate.