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 shoots straight out the far side scatters every which way — no special angle, no rainbow.
Bounces off the back
Light that bounces once off the back leaves the drop fanned into colors — and always near one 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
One rule makes a curve. The other makes a smear. 🌧️
Here's the whole rainy sky, and there's you, with your shadow on the ground. Each drop could send its colors to your eye. The big question is the rule: can a drop aim color at you from any angle, or only from one fixed angle? Guess first — then flip the rule and watch.
Guess before you flip the rule
Which rule makes a sharp curved rainbow instead of a smeared glow?