What causes double rainbows?

Regular rainbows occur when moisture in the air refracts
sunlight in such a way that it is broken up into its constituent
colours. The phenomenon occurs when the Sun is positioned
behind you and sunlight passes through the airborne water. The light
refracts (bends) inside the droplets and the white light is broken up.
Each colour has a diff erent wavelength so, depending on the angle
of refraction, a diff erent colour of light will be refl ected outwards; the
result of this process is what we observe when we see a rainbow.
Every rainbow is accompanied by another, secondary rainbow,
but it’s usually too dim to see. This double rainbow eff ect is due to
the continued refl ection of light inside each water drop. Sunlight is
actually refl ected twice inside a drop: once to produce the primary
rainbow and a second time at the back of the drop. This second
refl ection inverts the light but undergoes the same refraction, so
exits in the same way as before – though upside down. This second
refl ection reduces the intensity of the sunlight, but it also produces a
second inverted rainbow, creating a double arc of multicoloured light.