How Rainbows Form
How rainbows form: refraction, dispersion and reflection inside raindrops, why the colours appear in order, why rainbows are arcs and double rainbows.
Key takeaways
- Sunlight bends (refracts) entering a raindrop, reflects inside and bends again leaving it.
- Different colours bend by different amounts (dispersion), spreading white light into a spectrum.
- You see a rainbow with the Sun behind you and rain in front of you.
On this page
A rainbow is sunlight split into its colours by millions of raindrops. The physics combines three things: refraction, reflection and dispersion.
Step by step
- Refraction: sunlight enters a raindrop and bends as it slows down in water.
- Dispersion: different wavelengths bend by slightly different amounts, so white light spreads into colours.
- Reflection: the light reflects off the back of the drop.
- Refraction again: it bends once more as it leaves, spreading the colours further.
Light leaves each drop at roughly 40–42° to the incoming sunlight, with red at the larger angle and violet at the smaller.
Why the colours are in order
Each colour reaches your eye from drops at a specific angle. Red comes from higher drops and violet from lower ones, so red is on the outside of the arc and violet on the inside: red, orange, yellow, green, blue, indigo, violet.
Why rainbows are arcs
All the drops sending light to your eye at the same angle form a circle around the point directly opposite the Sun. The ground usually hides the bottom half, so you see an arc. From an aircraft, you can sometimes see a full circle.
Double rainbows
Light reflecting twice inside the drops creates a fainter secondary rainbow outside the first, with the colours reversed. The darker band between them is called Alexander’s band.
When to see one
Stand with the Sun behind you and rain in front, ideally in the morning or late afternoon when the Sun is low.
Related physics
Refraction follows Snell’s law. Why the sky itself is blue is a different effect, scattering; see why the sky is blue.
Each colour has its own wavelength and frequency; see wave speed, frequency and wavelength and the wider electromagnetic spectrum.
Frequently asked questions
Can you reach the end of a rainbow?
No. A rainbow’s position depends on where you stand, so it moves as you move.
Why are some rainbows brighter?
Larger raindrops and bright, low sunlight create more vivid rainbows.
Are there rainbows at night?
Yes. Moonbows form from moonlight, but they’re faint and often look white to our eyes.
Sources
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