Rainbow formation
Beginner
Refraction, reflection and dispersion in raindrops.
Sunlight is refracted into a raindrop, internally reflected once and refracted out, with deviation minimum at about 138°, so the bow appears at 42° from the antisolar point. Red on the outside.
Secondary bow at 51° with reversed colours.
The observer sees different drops for each colour.
In more detail
A rainbow forms when sunlight enters raindrops, refracts, reflects inside, and refracts again, dispersing into colours. The primary rainbow appears at about 42 degrees from the antisolar point.
Real-world uses
- Meteorologists use rainbows to understand droplet sizes.
- Physics teaching uses rainbows to illustrate dispersion and reflection.
- Rainbows appear when sunlight is refracted, reflected inside raindrops and refracted again as it leaves.
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Frequently asked questions
Why is a rainbow arc-shaped?
Because light emerges from raindrops at a fixed angle relative to the sun, forming a circle around the antisolar point.
Why is red on the outside?
Red light emerges at a larger angle than violet, so it appears on the outer edge.
Why is a rainbow curved?
The rainbow forms at a fixed angle of about 42 degrees from the shadow of your head, so the arc is set by the geometry.
Related topics in Optics
Applications of TIR · Prism and deviation · Dispersion · Scattering of light · Mirage · Refraction at a spherical surface