How does the slit width and the slit-screen separation affect the diffraction of light?
The angle of diffraction depends on the slit width and the wavelength. A diffraction minimum occurs at a given point on the screen when there is an integer-wavelength difference in path lengths to that point from opposite edges of the slit (see the ref.). Diffraction angle theta can range from 0 to 90 deg. For diffraction to occur within this range, the slit width must be >= wavelength. The formula is sin(theta) = m*lambda/a, where m is the order and a is the slit width. As 1st answer says, the screen distance only affects the linear distance of the minima from the slit and from each other.
