Until comparatively recently, it was believed that if one could find the exact spot at which the rainbow meets the ground, one would find there a hoard of buried treasure in the form of a pot, or crock, of gold. But no one ever did. The rainbow, alas, is almost an illusion, and its point of intersection with the ground does not exist.
To form a rainbow, millions of little raindrops act like tiny mirrors. But rather than being reflected on the outside surface, the light passes into each drop to be reflected from the concave surface at the back. Moreover, as the light passes into the drop, and also when it emerges from it to the air again, each concentrated beam of white sunlight is fanned out into all the separate constituent colours of the familiar spectrum.
Thus it happens that a complete kaleidoscope of separate colours - red, orange, yellow, green, blue and violet - leaves each single raindrop, and a similar spectrum of light leaves every other nearby raindrop. But as you watch a rainbow, from any one raindrop only a small part of this spectrum hits your eye; if your eye catches a beam of green light from a certain raindrop at the top of the arc, the blue light from that raindrop goes above your eye, and the red light from that same raindrop goes below.
You see a complete rainbow only because there are millions and millions of different raindrops.
Moreover, the band of thousands of raindrops that gives you green light would simultaneously give a band of blue light to anybody who might be suitably placed above you, and a band of red light to someone else below you. A different band of thousands of raindrops gives you the red light, and would give someone else blue light; and a third band of raindrops gives you the blue light - and so on.
Now what all this means, if you can follow it, is that any number of observers watching the same curtain of illuminated raindrops will each see their own different rainbow, using light from their own personal drop ensemble.
As a consequence, far from the rainbow being rooted in any particular "place" where the little people might have left a crock of gold, there are as many rainbows as there may be eyes to look at them.
Indeed, even your own two eyes are seeing two quite different rainbows, and if you are travelling in a car looking at "the" rainbow, what you actually see is a whole series of rainbows, one replacing the other in very quick succession.