Because the rays of refracted light, reflected upon other drops of rain, are again refracted, and then reflected again, forming a secondary bow.
489. Why are the colours of the secondary bow arrayed in the reverse order of the primary bow?
Because the secondary bow is a reflection of the primary bow, and, like all reflections, is reversed.
490. Why are reflections reversed?
Because those rays which first reach the reflecting surface are the first returned. If you hold your open hand towards the looking-glass, the light passing from the point of your finger will reach the reflector and be returned before the rays that pass from the back parts of the hand. Hence the image of the hand will present the reflection of the finger point towards the point of the finger.
491. Why are the colours of the secondary rainbow fainter than those of the primary?
Because they are derived from the refraction and reflection of rays which have already been refracted and reflected, and thereby their intensity has been diminished.
A lunar rainbow is caused by the light of the moon, in the same manner as the solar rainbow is caused by the light of the sun.