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.
Sometimes we can see a second, larger, rainbow outside the main one. This is called a "secondary" rainbow, and it is formed by rays of light that are refracted by the raindrop and then reflected inside the rain drop twice.
If you look carefully, you will see that the extra reflection means that the colours in the secondary rainbow are in the opposite order to the first (or "primary" rainbow). The secondary rainbow is also less bright because the light is being spread over a larger area of the sky.
The area between the two rainbows is known as 'Alexander's Band', named after Alexander of Aphrodisias who first described its occurrence in 200 AD.Moonbows Moonbow
Although quite rare, it is possible to see a rainbow at night. If the moon is shining brightly enough, light can be reflected through water droplets in the same way a rainbow is created. Because the moon is much less bright than the sun, 'moonbows' are much fainter than daytime rainbows.
This makes the colours difficult to see so they usually look white to the human eye, but you may be able to see the colours in a photograph taken with a long exposure time as in the example to the right.
Similar in appearance to rainbows, but different in their formation, you may have observed an upside-down rainbow or perhaps a bright circle that surrounds the sun. These are not rainbows but are examples of halos, formed by ice crystals high in the atmosphere.
Last updated: 25 November 2015