CBSE Class 10 Answered

The main reason for the formation of a rainbow is the refraction and reflection of the sun's rays as they fall on drops of rain.
As a ray passes into a drop of rain, the water acts as a tiny prism. The ray is bent or refracted as it enters the drop and is separated into different colours. Then it strikes the inner surface of the drop and gets further refracted or reflected and dispersed. Seven colours are thus formed and we rarely see not more than three or four colours due the blending of the adjacent colours. Each colour is formed by rays that reach the eye at a certain angle. This value is 42 degrees for a primary rainbow. The higher the sun, the lower the bow.
According to simple geometrical principles, all the rain drops which lie at this particular angle (or which subtend this particular angle at the obsever's eye) and in a direction opposite to the sun, lie in the form of a full circle or a part of it (arc). Even if there are enough drops to form a full circle, to an observer on the earth's surface, it will look like an arc only as it is limited by the horizon.