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Short · Light

Why is a rainbow always a circle?

Light refracts into a drop, reflects once and refracts out. Red arrives near 42 degrees from the antisolar point, so bright drops form a circle.

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Transcript

A rainbow is always part of a circle, centered on the shadow of your head. Each raindrop acts like a tiny prism. Sunlight bends going in, bounces off the back, and bends again coming out. Violet bends a little more than red, so red leaves at about forty-two degrees, violet at about forty. Now move a drop around the sky. Only drops forty-two degrees from that shadow send you red, and together they form a circle. The ground usually hides the bottom half. Try it yourself at Explainer Tools dot com.