A rainbow forms when sunlight enters raindrops, bends, separates into colours and reflects inside before leaving toward an observer. Its circular geometry means each viewer sees a slightly different bow.
A drop works like a small prism
Sunlight changes direction as it enters a raindrop. Different wavelengths bend by different amounts, separating the white light. The light reflects from the back of the drop and bends again as it exits.
Only droplets at particular angles send a given colour toward your eyes. A whole curtain of rain supplies many droplets at those angles, producing the arc.
The Sun must be behind you
A primary rainbow appears when the Sun is behind the observer and rain is ahead. The bow is centred opposite the Sun, so its height changes as the Sun moves. From an aircraft or a high viewpoint, more of the circle can become visible.
There is no fixed object in the sky that everyone shares; the rainbow is a viewing geometry between light, droplets and one observer.
Secondary bows reverse the colours
A secondary rainbow can form after two internal reflections. It appears fainter, higher and with reversed colour order. A dark band between the bows contains less scattered light and is known as Alexander’s band.
The familiar arc is therefore an optical process, not a physical bridge or a place where coloured material collects.
Sources and evidence
- NOAA NESDIS — What causes a rainbow? ↗
Refraction, reflection, dispersion and rainbow geometry.
- NASA — Rainbows and light ↗
Sun angle, droplets and atmospheric optics.
Sources consulted: 9 October 2026. Written with AI assistance and checked against the linked references; not independently reviewed by a subject specialist. Sources do not endorse this article. See our editorial policy.
Revision: Added original explanatory comparisons, source-linked evidence and a reader-focused example.