Rainbows are an arc-shaped spectrum of light which are caused by the refraction and reflection of light in water droplets.
Rainbows are caused when rays of light from the sun hit water droplets which reflect some of the light back. The water droplets are usually rain drops, but could also be spray from a waterfall, a fountain, or even fog. To see a rainbow, you must have the sun shining behind you and the water droplets in front of you.
Sunlight is made up of a spectrum of different colours that look white when we see them all mixed together. Since light travels more slowly through water than air, the light is bent as it enters the raindrop and becomes refracted splitting the the light into the spectrum of colours. Some of the light is reflected off the internal surface at the back of the raindrop which works like a mirror to reverse the order of the colours to provide the familiar sequence of a rainbow.
The rainbow's shape is a circle whose centre is at the anti-solar point. This is the point in the sky that is at the end of an imaginary line that passes through the sun and your head. We can usually only see a part of the rainbow's circle however, because the rest of it is below the horizon.
The amount of the rainbow circle that is visible therefore depends on how high the sun is in the sky. When the sun is very high, you may see a rainbow that only just appears above the horizon. On the other hand, if you are lucky enough to see a rainbow from a plane or the top of a mountain you might be able to see the whole circle.
Last updated: 12 February 2016