system
Given the following post, respond as if you're commenting on Reddit:
human
Why do we only see one big rainbow instead of myriad of very small ones? Every drop of water in the air kind of creates its own small rainbow by refracting the light, so why don't we see all of those individually?

Question has been answered, but feel free to add onto any dicussions or bring up points not yet mentioned.
gpt
A water droplet is a sort of mirror prism. A ray of light comes from the Sun, enters the droplet and changes its angle due to refraction, reflects off the inner wall of the droplet (at least partially), and comes out at a different angle. Different colours come out at different angles.

If you imagine a triangle from the Sun, to the droplet, to your eye, this is the path the light goes. You will get a specific colour if the angle between the Sun-droplet line and the droplet-eye line is just right. What this means is that all droplets in the same line-of-sight will give you the same colour, because the Sun->droplet->eye angle is the same along that line, and the light has been bent by the same angle for all those droplets.

A rainbow is (in principle) circular, centred on the point exactly opposite the Sun, because it doesn't matter which direction the light is bent, as long as the angle is right. So a full circle of lines-of-sights of droplets will cause rays of light of a specific colour to hit your eyes. Of course, the ground is in the way most of the time (the Sun is above ground, so the centre of the rainbow is below ground), and the rainbow ends up looking like an arc.

You also get double & triple rainbows etc because light can bounce more than once within a droplet, and will come out at a different angle after each bounce. But some light leaks out after each bounce, which is why the second + third + above rainbows are quite dim - and also why clouds aren't completely dark!