A gas absorbs a photon of 355 nm and emits at two wavelengths. If one of the emissions is at 680 nm, the other is at :

$\frac{1}{\lambda }=\frac{1}{\lambda 1}+\frac{1}{\lambda 2}$

$\frac{1}{355}=\frac{1}{680}+\frac{1}{\lambda 2}$

$\frac{1}{\lambda 2}=\frac{680-355}{680\times 355}$

$\Rightarrow \lambda 2=743nm$.