Dear all, I have pretty simple problem in terms of understanding but I can't write an analytical solution. Consider you have a laser beam (without divergence, single wavelength). You can see the intensity distribution on the screen perpendicular to the beam. Then you insert a thin glass layer which from side has some roughness at scales above (or much above than) wavelength: photons are simply scattered with certain angle. You can imagine this piece of glass as a distribution function of angles in space (we neglect even refraction). Problem: I need a function which shows me the intensity distribution at any distance after the glass layer. Problem 2: solve reverse case - we don't know angle distribution, but distributions (or just one distribution) after deflection rays by the glass layer. What equation should I write? Boltzmann probably doesn't fit as soon as integral over transversal space must be constant in time (number of particle with certain angular speed doesn't change over the propagation distance). And I can't solve continuity equation because velocity field is changed during the propagation... Thank you in advance!