Stupid Question ™
Dec. 16, 1999
By John Ruch
Q: Why do rays of sunshine breaking through the clouds sometimes appear to fan out instead of pointing straight ahead?
A: Called “crepuscular rays,” these sunbeams are basically individual quantities of sunlight that poke through or around some object, usually a cloud. They’re made visible by dust or vapor that scatters light from them to our eyes, and by the contrast with the cloud’s shadow.
Frequently, the rays project from a hole in cloud cover, appearing to spread out and widen as they go.
But that’s an optical illusion. In fact, the rays are straight and nearly parallel (within 0.5 degrees) with each other. That’s because they come from the Sun, which is very far away and much larger than the Earth. They’re a lot longer and bigger than they look—the bottom of a ray can be miles closer to you than the top, where it comes out of the cloud. (Some rays make it all the way across the sky.)
If you see the rays exactly sideways (a 90-degree angle), you’ll see they are parallel with each other, and slanted at whatever angle corresponds to the height of the Sun in the sky. (In our latitude, the Sun is never directly overhead.)
If you see the rays from any other angle, especially head-on, they’ll appear to “fan out.” By “fanning out,” we mean the rays seem to get wider (and farther apart from each other) the closer they are to us. Likewise, the rays seem to get smaller (and closer to each other) the farther they are from us.
In exactly the same way, if we look down train tracks, they appear to converge and finally disappear in the distance.
This illusion, called “perspective,” is based on the fact that light travels off objects in straight lines at definite angles. The farther away an object is, the smaller the angle of vision and the smaller the image it projects on your retina. As you look into the distance, the angle of vision gets smaller and smaller until it effectively reaches zero—the “vanishing point.”
These smaller images make the objects appear smaller, shrinking regularly with distance. If the object has straight, parallel sides, these will appear to converge with distance (unless you look at the parallel sides at a 90-degree angle, in which case they look normal).
Crepuscular rays are regularly shaped areas of sunlight with parallel sides (and also with a regularly-shaped, parallel-sided area of sky between any two given rays). The visible area of the ray becomes smaller with distance in a regular manner; thus, the parallel sides appear to regularly converge with distance. And of course, anything that appears to converge with distance will also appear to diverge (“fan out”) with closeness. Each ray appears to get wider, and each ray appears to get farther from the other rays, as they approach us.