If you look through a window screen at a distant street light, you will see the pinpoint of light broken into a cross like pattern of spots. The same effect can be obtained with a geologist's wire-mesh sieve and a pinhole light source behind it, as shown to the right of the page.

This is the phenomenon of diffraction. The wires of the screen scatter the light waves. These scattered waves reinforce in certain directions to produce spots of light, and cancel or interfere in other directions to produce darkness.

From the pattern of light and darkness in the diffraction pattern, a physicist can determine the spacings between wires in the original screen.