If you've seen a solar eclipse, you know that just before the sun is hidden from view something very strange happens. Shadows start swirling, as if the light from the sun is shining through a heat haze. The phenomenon is called "shadow bands."
Before the Eclipse
Very few things prepare you for the strange dimming that the world goes through during an eclipse, making everyone squint as though they're going blind. The sudden drop in temperature is pretty strange, too.
As the sun stops being a circle and becomes a crescent, shadows change. When you look at the shadows formed by the canopies of trees, or the slats in a bench or chair, you will see that the slivers of light between them have turned from rounded shapes to odd little crescents. If you've formed a pinhole lens with two index cards (the way they teach you in school) you'll see the rounded light from the pinhole change into a crescent as the sun forms the same shape. The slats between the bench and the points of light between the leaves are doing the same thing. The only difference is the shape of the "pinhole."
A full eclipse gets even weirder. Just before the sun completely disappears, the entire world starts waving. Waves of light and dark, like more-subtle versions of the light waves you see at the bottom of a swimming pool, travel across the entire visible world. Depending on the circumstances, these waves can be near stationary or they can be traveling fast.
Shadow Bands And Light Waves
Descriptions of shadow bands go way back, and there are a fair amount of drawings of them, but they are notoriously hard to study. They occur briefly, during a period of low light. They themselves can be very dim, and they move around, so they are hard to photograph.
Still, scientists believe they understand what causes the phenomenon. As the moon covers the sun, the sunlight stops coming from a "firehose" and gradually looks more and more like it's coming from a single point. It acts that way as well.
As light moves through the atmosphere, its journey is sometimes bent by the density of the air. Wind and heat can increase or decrease the air's density, in swirling patterns. (We can see these patterns when, for example, we see sunlight passing through a hot area, like the air over a grill or a baking hot road.)
When light waves bend, they interfere with each other, a peak and a trough canceling out in darkness, or two peaks becoming brighter. The "firehose" sends a flood of light down so we don't see these patterns, but the point source allows us to see them clearly.
A good way to test this idea out at home would be to light a candle, put it near a wall, and illuminate it with a flashlight. The heat from the candles will mess with the air density enough for the light from the flashlight to show up as swirls. The farther away you move the flashlight, making it more of a point source, the more clearly you should see the swirls.
But, when you do this, you won't see "shadow bands." You'll see a dot pattern, not a wavy pattern. Shadow bands aren't the result of a strict point source. The sun doesn't become a point until the final moment before the eclipse. When the sun is getting eclipsed, it becomes a thinner and thinner crescent — the same crescent that made the crescent shaped lights between the leaves in the picture in the last section. So rather than being the interference pattern of a bunch of dots, shadow bands are the result of the interference pattern of a bunch of crescents.
To make shadow bands with a flashlight and candle, then, you'll have to "eclipse" the flashlight with a round piece of cardboard.
Note: Many thanks to Dr. Wolfgang Strickling, for the photo and gif of the shadow bands. Check out more of the images at his site, Shadow Bands During a Total Solar Eclipse.