1. The problem statement, all variables and given/known data A collimated beam from a white-light source is incident normally on a transmission grating with 500 lines per mm. The transmitted light then passes through a lens which is used to project the visible (380–780 nm) spectrum of the light source on to a strip of photographic film and to just cover its length of 35 mm. Calculate the focal length of the lens 2. Relevant equations The diffraction grating has a pattern where fringes are found through nλ = dsin(Θ) I also know the thin lens equation: 1/o +1/i = 1/f o is the object distance, i is the image distance and f is the focal length of the lens. 3. The attempt at a solution I think I'm not understanding the question properly. Light goes in, is diffracted, some of it is collected by the lens (at unknown distance from the grating) and then it's shone on to a 35mm square. I don't know how I can work out where anything is or what sort of distance scale is involved.