system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why rainbows are arched and what factors decide their dimensions?

I have always been told that a rainbow is caused by the sun shining through rain drops which I've never really questioned. But when I saw a rainbow yesterday I wondered why the whole sky isn't multi-coloured, and what decides the thickness of the colour bands and the overall dimensions of the rainbow itself? Are they all the same size / heights or is there ones that stretch for miles and other really small ones?
gpt
You  see a rainbow when you, the raindrops and the sun are all in a  certain orientation to each other.

The rainbow is _really_  a halo around the lightsource - the  sun.  However, much of halo is below the horizon.  So...it's not  REALLY a bow, it's really just a  part of  a circle.  That  "circle" is  all the spots  where the light is at 42  degree angle from the raindrop and the incoming light  of the  sun.