Diffraction of Light

When a narrow single slit is used to replace the double slit, what we see on the screen is not the image of the single slit; instead, there are parallel bands. The brightness of the central band is about twenty times greater than the first bright band on either side.

Consider point A1 at the edge of the slit and point A2 at the center of the slit. If point P on the screen is in the middle of a dark fringe, the length difference between the two light paths A1 P and A2 P should be equal to one half wavelength.

The width of the slit b is supposed to be small

in comparison with the distance between the

slit and the screen, L. Thus, the dark point is at

sinq = x / L = (?l / (?b) = l / b