Sharp Shadows...

This time of year, when the sun is high in the sky, is a good time to observe an interesting phenomenon: unusually sharp shadows on a wall. Normally the sun's breadth causes a “fuzziness” around the edges of a shadow. However, sometimes you'll see shadows whose sharpness really stands out. My favorite examples of these are shadows of plants (especially grass seed heads), where you can see very fine details in the shadows.

What causes these extra-sharp shadows?

There are many mechanisms, but all of them boil down to one thing: something has caused the sun's light to become (or approximate) a “point source”. This term from the optical sciences just means that a source of light emanates from a single point (rather than from a broad disk, like the sun's light normally does).

One way to get a point source is from a small hole in an opaque barrier. I've seen good examples of sharp shadows that resulted from the inadvertent folds of a drape, making a pinhole that the sun shines through. But the very best point sources come from reflection off a convex (bending outwards) surface – and these are something very commonly found almost everywhere: car windshields. Even better are the little “wide angle” mirrors that people stick to their rear-view mirrors – these have a very strongly curved surface and thus are closer to a point source.

My favorite examples of sharp shadows have come from car mirrors or windshields reflecting lights through plants, and then through the window of a room that is darkened because there's no direct sunlight coming into it. The office building where I work has just such a room (an office on the south side of the building that is protected from the sunlight by an overhang). My home's north-facing windows can provide the same effect, if I've parked my truck in just the right spot.

Have you ever seen these extra-sharp shadows? If so, did you figure out what caused them?