Sophisticated Shadows

There’s really no end to the discoveries possible at this exhibit. But there’s one phenomenon that most people find delightfully baffling: Hold up a screen with holes of various shapes punched in it—a circle, a square, a star—and the patterns of light and shadow made by each aren’t at all what you’d expect. You don’t see a circle, a square, or a star—but instead an upside-down image of the letter F—which happens to be the shape of the light source.

Light passing through small holes or gaps behaves like light in a pinhole camera—light rays from the source cross and invert as they make their way through the hole. Provided the light source is far enough away, the exact shape of the hole doesn’t matter.

Similar optics are in play when you see dappled circles of light beneath a leafy tree. Those perfect circles of light don’t come from perfectly circular gaps between the leaves—they are images of the circular Sun.

During an eclipse, it’s easy to see that light passing through a small hole takes the shape not of the hole, but of the light source. (click image to enlarge)