(i) Statement of Bohr's quantization condition de- Broglie explanation of stationary orbits (ii) Relation between $\lambda_1, \lambda_2 \lambda_3$ (i) Only those orbits are stable for which the angular momentum, of revolving electron, is an integral multiple of $\; \frac{h}{2\pi}$ .[Alternatively $L = \; \frac{n h}{2\pi}$ i.e. angular momentum of orbiting electron is quantized.] According to de-Broglie hypothesisLinear momentum $(p) = \; \frac{h}{\lambda}$And for circular orbit $L = r_n p$ where ' $r$ ' is the radius of quantized orbits. $= \; \frac{r h}{\lambda}$Also $L = \; \frac{n h}{2\pi}$ $\therefore \;\; \frac{r h}{\lambda} = \; \frac{n h}{2\pi}$ $\Rightarrow 2\pi r_n = n\lambda$ $\therefore$ Circumference of permitted orbits are integral multiples of the wave-length $\lambda$. (ii) $\; E_C - E_B = \; \frac{h c}{\lambda_1}$ .......(i) $\; E_B - E_A = \; \frac{h c}{\lambda_2}$ .......(ii) $\; E_C - E_A = \; \frac{h c}{\lambda_3}$ .......(iii) Adding (i) & (ii) $E_C - E_A = \; \frac{h c}{\lambda_1} + \; \frac{h c}{\lambda_2}$ ........(iv) Using equation (iii) and (iv)$\; \frac{h c}{\lambda_3} \; = \; \frac{h c}{\lambda_1} + \; \frac{h c}{\lambda_2}$$\Rightarrow \;\; \frac{1}{\lambda_3} = \; \frac{1}{\lambda_1} + \; \frac{1}{\lambda_2}$