1What a raindrop does to sunlight
White light hides colors — and a drop splits them out
You only need two ideas. Watch each one:
White light is colors stacked up
Sunlight looks white, but it's really every color bundled together. Bend it and the colors fan apart, because each color bends by a slightly different amount.
A raindrop is a tiny mirror-ball
Light dives into a round drop, bounces off the back, and comes back out. On the way in and out it bends — so it leaves as a little fan of split colors.
2Two paths light can take
Straight-through light vs the bounce-back ray
Light hits a drop and can go two ways. Only one of them builds a rainbow:
Just passes through
Light that passes through without an internal reflection does not form the primary rainbow seen opposite the Sun.
Bounces off the back
Light that bounces once off the back leaves the drop fanned into colors — with many of the rays clustered near a special angle.
3Your turn — aim a sunbeam at one drop
Slide where the light hits and watch the colors come out
Move the sunbeam up and down across a single drop. Watch it dive in, bounce off the back, and leave as split colors. The exit angle changes as you slide — but aim near the edge and the colors crowd together and pile up toward one special angle (about 42°). That bunching is exactly why the bow glows brightest there.
4Now fill the sky with drops
A new viewpoint: which drops will send you color?
Keep the Sun and the field of drops fixed. Now take a step sideways. Use what you found about the angles from one drop to predict what a new viewpoint will change.
Guess before you take a step
When you step sideways, will exactly the same drops still send the bright rainbow colors to your eye, or will a different set light up?