How raindrops separate sunlight into a rainbow
A rainbow forms when sunlight bends as it enters raindrops, reflects inside them and bends again on leaving. Different colours bend by different amounts, so they emerge in different directions. You see an arc because only droplets at particular angles send each colour towards your eyes.
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Colour depends on the viewing angle
A primary rainbow involves light entering a water droplet, reflecting once inside and leaving through another part of its surface. Refraction at the boundaries changes the light's direction.
Water bends different visible wavelengths by different amounts. This dispersion separates sunlight into colours that leave the droplets at slightly different angles.
The visible bow is centred on the direction opposite the Sun. Droplets positioned at the right angles send particular colours towards your eyes, producing an arc with red on the outside and violet on the inside.
- The Sun is behind the observer.
- Sunlit droplets are ahead.
- The viewing geometry selects the light that reaches the eye.
A rainbow is therefore not a fixed coloured object hanging at one location. Moving changes which droplets supply the light you see.