In the darkest of nights, you won’t be scared, UNTIL the red pigeon arrives
This answer is purely fictional:
Imagine all the rain drops forming a film of glass. Now this glass is curved like a cylinder since rain is basically water flowing from a pipe in the sky. So you are looking at the sun through a curved glass hence you see the curved bow. Now your eyes are spherical in shape, hence the light reflected off the cylinder will appear in colours. Go outside and look at any cylinder, you’ll see rainbow colours.
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Wow this is really cool. Thanks
https://www.youtube.com/watch?v=RKT-sKtR970&t=309