Science
Why does a rainbow always appear opposite the Sun?
- Light bounces back out of raindrops towards you
- Rain falls that way
- The Sun is behind the rain
The answer is Light bounces back out of raindrops towards you. Light enters a drop, reflects, and leaves at about 42 degrees.
Because you are looking at light that has gone into raindrops and come back out towards you. It enters the drop, bends, reflects off the inside of the far surface, and leaves - and the geometry of that path sends it back at about 42 degrees from the direction it came. So the bow is always centred on the point directly opposite the Sun from your eye.
That means a rainbow is not in a place. Every observer sees a different one, made by different drops, and it moves when you do, which is why you cannot reach the end of it. It is also a full circle - the ground simply cuts it off, and from an aircraft you can sometimes see the whole ring.
The colours separate because each wavelength bends by a slightly different amount. The fainter second bow comes from light reflecting twice inside the drop, which exits at about 51 degrees and with the colours reversed - red innermost. Between the two the sky is noticeably darker, since no light leaves the drops at those angles; it is called Alexander's band, after a Greek who described it around AD 200.