Rainbows are caused by the splitting of white sunlight into it component colors
by raindrops. Some of the light that falls on a water drop enters the drop.
As it enters the drop, the color components of the sunlight are refracted (bent) by different amounts
depending upon their wavelength (we perceive the different wavelengths as different colors.)
Then, the different colors reflect off the back of the inside of the drop, and
when they pass through the front of the drop again, they are refracted once again.
A rainbow is always directly opposite the sun from the observer's perspective. This explains why rainbows are only seen when the sun
is low in the sky, usually in the late afternoon (in which case the rainbom will be seen to the east of the observer), or early morning
(in which case the rainbow will appear to the west of the observer).
If the observer could see the shadow of his head cast by the sun, it would be in the exact center of a circle
where the rainbow forms the top portion of that circle.
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