Rosehip Mole asked, and Nib — askNib's tutor — drew the answer live at a whiteboard. This is the spoken transcript; enable JavaScript to watch it drawn.
What makes rainbows appear?
Rainbows appear when sunlight meets raindrops floating in the air after a storm — light and water together make the magic.
White sunlight looks like one color, but it's secretly a mix of many colors bundled together.
When light enters the raindrop it slows down and bends — that bending is called refraction, and it splits the colors apart.
Inside the drop, light bounces off the back wall like a tiny mirror, then bends again as it exits — sending each color out at its own angle.
Millions of raindrops each send out their own color to your eye, stacking up red on top through violet on the bottom — a rainbow arc.
So a rainbow is just sunlight taking a detour through raindrops, splitting into all the colors it was hiding.