Science Questions

Do rainbows follow the contours of the horizon?

Sun, 10th Jun 2007

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Question

Alan, Kent asked:

Do rainbows follow the contours of the horizon, so the curvatire of the Earth gives the curve of a rainbow, or is it the sun? Can you also get a Moonbow?

Answer

Itís actually neither.  A rainbow is caused by sunlight hitting clouds or rain, and can usually only be seen if there is a background of dark clouds.

Sunbeams go into a raindrop, reflect off the back inside surface of the raindrop and come back out of the front.  When they come back out, the raindrop behaves like a prism, and splits the light into all the different wavelengths that make up white light.  White light, such as light from the Sun, consists of lots of different colours of light added together, so when they are all together it looks white, but if you split them up you can see the colours.

A double rainbow occurs when the light does a double journey; it reflects off the back of the raindrop, then bounces off the front inside surface, back to the back surface and then out.  This causes a second rainbow outside the first one, but with the colours the other way around.

The curvature of a rainbow different.  If you could look at a rainbow from far enough away, you would see a complete circle.  This is caused by the angle at which light exits a (roughly spherical) raindrop.  We canít see the entire circle because the Earth gets in the way.

You do get some similar effects from the moon.  When you get very high altitude clouds consisting of many ice particles, you can often get a similar effect to a rainbow.  Itís less common and less bright than a rainbow though.

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