Why do we sometimes see a rainbow after rain, always opposite the Sun?
Sunlight enters each raindrop, disperses into colours (different wavelengths bend by different amounts), reflects off the back of the drop by total internal reflection, and disperses again on the way out.
Sunlight enters each raindrop, disperses into colours (different wavelengths bend by different amounts), reflects off the back of the drop by total internal reflection, and disperses again on the way out. The geometry sends the light back at about 42° to the incoming sunlight, so you see the arc opposite the Sun, red on the outside and violet on the inside.
Key point
Rainbow = dispersion + internal reflection of sunlight inside water droplets.
Common mistake
thinking a rainbow is light simply reflecting off rain like a mirror.
Memory tip
Sun behind you, rain in front — dispersion paints the arc.
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