Transitions, Photon Emission & Excitation question from MHT CET 2026 (12 May, Shift 1)

Answer: (a)

1. Emitted frequency $\nu \propto \left(\frac{1}{n_f^2}-\frac{1}{n_i^2}\right)$; emission needs a jump from higher $n$ to lower $n$.
2. (b) and (c) are absorptions. (a) $5\to2$: $\frac14-\frac1{25} = 0.21$; (d) $3\to2$: $\frac14-\frac19 = 0.139$.
3. So the answer is (a) $n=5$ to $n=2$.