Problem: A monochromatic source with wavelength lambda and amplitude E0 is located behind a wall with two very narrow slits. The separation between the slits is d. We would like to calculate the intensity I(k(transverse)) on a screen located z = L where L >> d. Solution: We can consider the two slits as two monochromatic sources. The outgoing field from each slit can be approximated (for simplicity) by a wave from a point source with amplitude E0 and wavelength lambda. At each point of the screen the intensity depends on the total field, i.e. the sum of the two fields.. after calculating the sum we get an expression which is a function of the transverse momentum.. Could anyone explain what is the exact connection (in the physical manner of course) between the transverse momentum and the intensity?