One of the spectacular atmospheric phenomena is the formation of rainbow during rainy days. The rainbow is also an example of dispersion of sunlight by the water drops in the atmosphere.
When sunlight falls on small water drops suspended in air during or after a rain, it suffers refraction, internal reflection and dispersion.
If the Sun is behind an observer and the water drops infront, the observer may observe two rainbows, one inside the other. The inner one is called primary rainbow having red on the outer side and violet on the inner side and the outer rainbow is called secondary rainbow, for which violet on the outer side and red on the inner side.
Fig. shows the formation of primary rainbow. It is formed by the light from the Sun undergoing one internal reflection and two refractions and emerging at minimum deviation. It is however, found that the intensity of the red light is maximum at an angle of 43o and that of the violet rays at 41o. The other coloured arcs occur in between violet and red (due to other rain drops).
The formation of secondary rainbow is also shown in Fig. It is formed by the light from the Sun undergoing two internal reflections and two refractions and also emerging at minimum deviation. In this case the inner red edge subtends an angle of 51o and the outer violet edge subtends an angle of 54o. This rainbow is less brighter and narrower than the primary rainbow. Both primary and secondary rainbows exhibit all the colours of the solar spectrum.
From the ground level an arc of the rainbow is usually visible. A complete circular rainbow may be seen from an elevated position such as from an aeroplane.