Problem 5.140

Light falls normally on a transparent diffraction grating of width $l=6.5 \mathrm{~cm}$ with 200 lines per millimetre. The spectrum under investigation includes a spectral line with $\lambda=670.8 \mathrm{nm}$ consisting of two components differing by $\delta \lambda=0.015 \mathrm{nm}$. Find: (a) in what order of the spectrum these components will be resolved; (b) the least difference of wavelengths that can be resolved by this grating in a wavelength region $\lambda \approx 670 \mathrm{nm}$.