Diffraction: Turning a Corner

The trick that light sometimes plays with shadows, giving them bright bends inside their edges, is caused by diffraction—the result of light’s traveling in waves.

If you wiggle a stick in water, a series of waves will flow out in all directions. When they encounter an obstacle, like a plank sticking out of the water, the part of the wave that strikes the very edge of the plank sets up a new series of ripple from that point. Some of the ripples will fan out around the edge—in effect, turning the corner.

That is also what light does. When the waves from a distant pinpoint of light strike an opaque object, they bend around the edges, curving both into the shadow and into the path of other waves from the same light source. Waves bending behind the object create a bright line where the shadow would ordinarily begin. But the waves moving in the opposite direction overlap opposing light waves. Where the crests of the waves meet, they tend to reinforce each other and create bright lines. But where crest meets through they cancel each other, and dark bands result. This overlapping is visible in the picture of a shadow on the opposite page, which has bright edges and distinctive patterns of alternating light and dark bands.


Life Science Library - Light and Vision