A rainbow is caused by three
important optical effects: reflection, refraction, and dispersion,
all working together.The rainbow forms when sunlight passes
over your head and illuminates falling raindrops in the
sky in front of you. This sunlight enters each spherical
raindrop, partially reflects from the back surfaces of the
raindrop, and then leaves the raindrop and heads toward
you. The raindrop helps some of the sunlight make a near
U-turn. But the path that the light follows after it enters
the raindrop depends on its color.
The colored rays of light then partially reflect from the
back surface of the raindrop because any change in light's
speed also causes partial reflection. Now the various colors
are on their way back toward you and the sun. The light
bends again as it emerges from the raindrop and the various
colors leave it traveling in different directions.
Only one color of light will be aimed properly to reach
your eyes. But there are other raindrops above and below
it that will also send light backward and some of that light
will also reach your eyes. But this light will be a different
What you see when you observe the rainbow is the lights
that many different raindrops send back toward your eyes.
The upper raindrops send their red light toward your eyes
while the lower raindrops send their violet light toward
your eyes. You see a series of colored bows from these different
Occasionally more than one rainbow is seen, the main or
primary is sometimes accompanied by a fainter and higher
secondary rainbow. The less frequently seen rainbow is caused
by double internal reflection. This results in an arc whose
vertical color sequence is the inverse of the primary rainbow's.
We generally see only rainbow arcs, because the formation
of water droplets is cut off at the ground. If we were on
a cliff, we could see a complete circular rainbow. Also
the higher the sun is in the sky , the less of a rainbow
we will be able to see from the ground.
If you are still unclear about the concepts, please refer
the animation below.