Solution to task 511 from Chertov´s for extramural stud

Solution to task 511 from Chertov´s for extramural stud

511. What is the smallest number Nmin of grooves that a diffraction grating should contain, so that in the second-order spectrum two yellow sodium lines with wavelengths λ1 = 589.0 nm and λ2 = 589.6 nm can be seen separately? What is the length l of such a grating if the grating constant is d = 5 μm?