Solution to task 514 from Chertov´s for extramural stud

514. Normal white light is incident on a diffraction grating containing n = 600 lines per millimeter. The spectrum is projected by a lens placed near the grating onto the screen. Determine the length l of the first-order spectrum on the screen if the distance from the lens to the screen is L = 1.2 m. The boundaries of the visible spectrum: λcr = 780 nm, λf = 400 nm.