What you Need
What to do
On a sunny day find somewhere on the edge of the shade, and stand with your back to the sun.
Spray water into the sunny area and look through the spay into the shady area - othewise any effect will be washed out by the bright background. You may have to move your head around a bit.
What can you see?
Try moving your head, does the effect stay still relative to the world the water droplets or you?
What may happen
If you get it right you should find that you can see a rainbow, and as your head moves the rainbow moves with you.
Why does it happen?
You are making a rainbow in exactly the same way that nature does it, by shining sunlight onto water droplets. In a rainbow these water droplets are of course raindrops.
White light from the sun is made up of many different colours, and when the light hits an individual raindrop the light is refracted (bent) as it enters the drop because light travels more slowly in water than air, some of it is reflected from the back of the drop and then is refracted again on the way out.
Water refracts different colours different amounts so they end up leaving in different directions. You can see this effect if you look at the reflection of a light in a glass, just before the reflection disappears it changes colour at an angle of about 40° to the incoming light.
A raindrop is of course a sphere so the light is actually reflected out in a way that is symmetrical around the incoming sunlight - it is reflected in a series of cones. This means that if you look at the raindrop from different places it looks different colours.
So if you view many water droplets you will see different ones from different directions, so they appear different colours. If you see millions of droplets, in some directions all of them will appear coloured - you will see a rainbow.
Have you ever seen a circular rainbow? I saw one while riding in an airplane. I was looking toward the ground and noticed the shadow of the plane racing along the ground. A rainbow appeared in a perfect circle, with the plane's shadow in the center. That sight was worth the pot of gold I paid for the plane ticket! Mike, Mon, 26th Apr 2010
that sounds cool. Amazing nature johnttrick, Tue, 31st Aug 2010
That sounds about right, your shadow should always be at the centre of any rainbow, just normally there is not enough rain between you and the rainbow below you so you can't see it. daveshorts, Thu, 2nd Sep 2010
Full circle rainbow http://atoptics.wordpress.com/2011/06/07/full-circle-rainbow/ solarhalo, Sat, 25th Jun 2011