What determines the size of a rainbow

asks rtell:
A. First you need to understand how rainbows are formed. Rainbows need strong sunlight and water droplets suspended in the air, which is why you see them on 'April showers' types of days, where showers are quickly followed by blue skies. When the sunlight passes through a water droplet, it is refracted. This changes the angle that the light travels when it enters and leaves the droplet, depending on the wavelength of the light rays.

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Q. What determines the wavelength
A. The colour of the light. The sun produces light which is composed of a mixture of colours: red, yellow, green, blue, violet. Each of these interacts in a different way with water molecules. The water droplet then changes the angle of each colour and this makes them appear as separate colours.

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Q. So what makes rainbows the size they are
A. There are natural rules which govern the angle between you (the observer), the sun and the rainbow, but not its exact position or size. That's why if you watch a rainbow through the window of a moving car, it appears to move with you. The size is a matter of perception.

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Q. How does that work
A. We know that things which are far away can be concealed by much smaller thing which is nearer - a nearby tree can conceal a far away mountain, for example, but we know that the mountain is a much bigger object.

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When we look at a rainbow, this works in reverse. Your brain measures the angle of the rainbow and sees it along with other features, such as mountains and clouds. If these objects are far away, your brain interprets the rainbow as a large thing. If, however, the surrounding features are nearer, the rainbow appears to be closer to you. It's the same effect you get when you look at the moon - it appears to be huge when it's close to the horizon, but tiny when high in the sky.

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By Sheena Miller