The Shadow Illusion

Cut any shape hole into a piece of cardboard and hold it up to the sunlight.

When sunlight shines through this hole, it casts a circular image on the wall regardless of the shape of the hole.

Why does this happen?

The size of the hole also doesn't affect the size of the image.

This counterintuitive demonstration shows that the hole is acting like a pinhole camera, producing an image of the sun on the wall. Therefore the size and shape of the hole have no effect on the size and shape of the image.
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https://www.youtube.com/watch?v=R8oiho_gKSo