Rainbows

[Henryn]

Why is a rainbow curved?

[Isaacr]

Raindrops are spherical and therefore symmetrical. Light passing through a drop is diffracted like light passing through a prism, but because the drops are symmetrical, light is scattered forming a circle of colours, hence the curve. If it wasn't for the ground getting in the way we would see the whole circle, not just a semi-circle, and in the right conditions complete circular rainbows can be seen, they're called halos.