The magic behind a rainbow

Who doesn't love the beauty of a rainbow?

At BBC Weather Watchers HQ we're often inundated with rainbow photos, but do you know why this spectacular arc of colour forms?

Rainbows need both moisture and sunshine, so you won't see a rainbow unless the sun is out. A rainbow is an excellent demonstration of the dispersion of light, and proof that visible light is made up of a spectrum of wavelengths, each associated with a distinct colour.

Water is denser than air, so when sunlight passes through the air to a raindrop it slows and changes direction (or bends). Physicists call this process refraction. Each individual droplet of water acts as a tiny prism that both disperses the light and reflects it back to your eye.

When sunlight is refracted by the droplet of moisture, the observer - standing with the sun behind them - will see an array of colours in an arc as the refracted sunlight is reflected back.

In theory, every rainbow is a circle, but from the ground usually only its upper half can be seen. You could see a full circular rainbow if you were looking from a high point - such as a building, mountain top or from an aeroplane.

Rainbows witnessed at dawn and dusk can, on occasion, be pink and this is because of the low sun angle at that time of day. At sunrise and sunset, the sunlight has to travel through a thicker slice of atmosphere and so much of the shorter wavelength light has been scattered by the time it reaches our eye. This leaves mostly the red end of the spectrum to be refracted and reflected - hence the pink appearance.

You may be surprised to hear that double rainbows are actually quite common - they occur because the sunlight is reflected twice inside the raindrop. This also means the sequence of the colours is reversed, so look closely next time you see one.

Happy rainbow spotting!