See exactly why rainbows are always at 42° from the anti-solar point, why colours always appear in the same order, and why you can never walk to the end of a rainbow.
White sunlight enters a spherical raindrop, refracts at entry, reflects off the back, and refracts again at exit. Different wavelengths refract at slightly different angles (dispersion), spreading red (~42°) from violet (~40°) into an arc.
Adjust the sun angle to change where the rainbow appears. Use the raindrop size slider to see how drop size affects colour purity. Toggle the secondary rainbow for the reversed colour order and dimmer outer arc.
A rainbow is actually a full circle — the ground cuts the bottom half off. From an aeroplane you can see the complete circular rainbow below you. Every person sees their own unique rainbow, centred on the shadow of their own head.