Hi, I have a major lack of understanding regarding this one issue. The following is consequently pretty incoherent, so I would be even more grateful to anyone who bothers to read and reply. I can derive the intensity distribution for, say a diffraction grating or single slit, in the Fraunhofer limit. However I really don't understand what happens when a lens is put in the mix. For example, a diffraction grating with a lens right next to it (not on the same side as the source), observing the pattern in the focal plane: does the intensity distribution have the same angular dependence as before, but with respect to the lens axis? What about the picture attached (where the focal length is 2.5mm). I can derive, in the Fraunhofer limit, the intensity as a function of angle with respect to the central normal (or perhaps any normal?) to the grating, but have no idea how to get the intensity in the focal plane of the lens (at 7.5mm). How do I find the positions of the maxima and minima? (I'm thinking maybe the central maximum would be on the optical axis, as the normal to the grating is parallel to the axis and so is directed onto the focal point: but how do I know the separation? Is it the same angular dependence as without the lens, but now with respect to the lens axis?). Sorry for the rambling, any replies would honestly be very much appreciated.