A sodium lamp emits two close wavelengths

PHYSICS · Subjective · A Level · Physics · Waves · Waves

Problem

A sodium lamp emits two close wavelengths,

589.0\,\text{nm}

and

589.6\,\text{nm}

. The light passes through a diffraction grating with

400\,\text{lines\,mm}^{-1}

and falls on a distant screen. Calculate the angular separation (in degrees) between the third–order maxima of the two wavelengths.