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Can You View a Round Solar Eclipse Through a Square Hole?
Here’s a cool way to watch the eclipse on Monday—and learn about the weird physics of light while you’re at it.
If you stand under a tree, the small spaces between the leaves will act as pinholes to project a bunch of little crescent images. I actually have a card from PUNCH(Polarimeter to Unify the Corona and Heliosphere) that demonstrates this with three holes—circular, triangular and square. Even if you can't make it to a location with a total eclipse, you can still have a ton of fun with a pinhole.
Or read this on Wired