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
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
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.