Why do rainbows form as curved arcs?
Quick Answer
Rainbows are made when sunlight is refracted, internally reflected, and dispersed by water droplets. The circular shape is created by geometry, but from the ground we usually see only an arc.
The Full Story
Each raindrop acts like a tiny prism and mirror at the same time. Sunlight enters the droplet, bends, reflects inside it, and bends again as it exits, splitting white light into colors. The angle for the brightest primary rainbow is about 42 degrees from the direction opposite the Sun, which is why rainbows appear in a circle around that point. From the ground, the Earth blocks the lower part of the circle, so we usually see an arc. In an airplane, people can sometimes see a full circular rainbow.
Key Facts
YouTube Angle
Suggested video title for this topic:
"Why is a rainbow really a circle?"
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