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After a rain in the fore noon or after noon when the sun rays are making  angle of 40 to 42 degrees with the horizontal then we have a possibility of  primary rainbow.

See diagram. 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. 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.

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.