Why is a rainbow always a circle?

A rainbow is not an object sitting over one hill. It is a direction: light arriving from drops at a particular angle around the line from the Sun through your head.

▶ Try it yourself in Rainbow Lab: raise the Sun or reveal the full circle from a plane.

What happens inside one drop?

Sunlight refracts as it enters, reflects once from the back, then refracts again as it leaves. Dispersion separates colors: the strongest red rays emerge about 42° from the antisolar line and violet near 40°.

Where does the circle come from?

Every drop 42° from that line can send red to your eye. Rotating that direction makes a cone; the lit drops appear as a circle centered on the shadow of your head. The ground normally blocks the lower part. At a Sun height of 42°, the outer bow is at or below the horizon.

The model omits secondary bows, supernumerary bands, polarization, terrain and drop-size effects. Photos are AI-generated illustrations.