Question: What determines the size of a rainbow? They obviously vary as shown by double rainbows.
Answer: The size of a rainbow is fixed by the way the Sun's rays go through the raindrops. When a light ray strikes a raindrop, part of it is reflected and lost and part is refracted into the drop. When this ray hits the back of the drop, part of it is refracted out and part is reflected back to the front surface. Part of this reflected ray is again reflected (see later) and part is refracted back out.
If the original ray hits near the centre, it will be deflected by 180° and return along the same path. This is how catseyes work, but you will never see sunlight reflected this way because of the shadow cast by your head.
But what happens if the original ray hits the raindrop off-centre? As the ...
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