Question

A source containing a mixture of hydrogen and deuterium atoms emits light containing two closely spaced red colors at  = 656,3 nm whose separation is 0.18 nm. Find the minimum number of lines needed in a diffraction grating, which can resolve these lines in the first order.

Answer 1

The number is

$$n=\frac{\lambda}{m\Delta \lambda}=3646\text{ lines}.$$