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Looking at the shade under a tree, why does the light form circles?

Answer:

The gaps between the leaves act as pinholes, producing an image of the sun on the ground. This is really a marvelous feat by the tree, placing leaves to capture all but the tiniest pinholes of sunlight. I have seen the ground covered with crescents during a partial eclipse. If a plane flies over, you can sometimes see the shape of the plane in the circles.