How do 2 waves in a double slit get cut? I know that light spreads in all directions so, when a double slit is infront of a light source, then it only allows light that goes through the slit past the wall. Why do interference patterns depend on the wavelength? Isnt the wave length perpendicular to the orientation of the slit? I would image the interference patters depend on the angle with which the light gets cut, but I don't see conceptually how it depends on the wavelengths. Can someone explain this conceptually?