fraunhofer diffraction

sachi

We have a convergin lens with focal lens f. A screen with a gaussian transmission function is placed f/2 to the right of this lens. and at a further distant of f/2 from the screen we have the focal plane of the lens. We need to find the form of the observed pattern in the focal plane of the lens. n.b we have incident plane waves. I know that we have a virtual image in the focal plane of the lens, and this will act as a virtual object for the screen, but I'm not sure what to do next.