Rainbow occurs due to the phenomenon of dispersion of white light from Sun when it comes out of water droplets suspended in air hundreds of meters above Earth's surface.
After a rain in
the fore noon or after noon when the sun rays are making an angle of 40 to
42 degrees with the horizontal then we have a possibility of primary
rainbow. There are millions of rain drops suspended in air after a rain above
First the Sun rays are
refracted in to water drops suspended in air some hundreds of meters in to the
sky. Then those light rays are totally internally reflected and then
refracted to reach the human eye at a cone angle of 40 to 42 degrees. The
refraction of white light at the exit from rain drop results in a dispersion.
Red color is visible on
top and violet is on the bottom. The dispersion of light takes place at the
last refracting surface of water drops. This is visible to us as the VIBGYOR
color bands in a semicircular arc in the sky. Actually the rainbow is a full
circle within the cones angles of 40 to 42 degrees.
A secondary rain bow is also
formed with violet on top and red below other colors. It is fainter than the primary rain bow. It is due to two total internal
reflections before the light emerges out of the water drops with refraction.
The refraction gives rise to dispersion of white light.