How Is A Rainbow Formed?

Question: How Is A Rainbow Formed?

Answer: The birth of each rainbow begins with millions of tiny rain droplets, without them – a rainbow would not occur. Rainbows can be observed whenever there are water drops in the air and sunlight shining from behind the observer at a low altitude angle.

Because of this, rainbows are usually seen in the western sky during the morning and in the eastern sky during the early evening.
The rain droplets serve as a type of reflector of light. White light enters one individual rain droplet and exits as one specific color of the spectrum. If you only had a few rain droplets you would only see a few colors. This is typically why rainbow appear after a rain storm.

Each rain droplet has a function in the formation of the rainbow. Sunlight enters the rain
droplet at a specific angle and the rain droplet separates the white light into many different colors.
This angle is a fixed measurement between your eye and the sun.

See also: Scattered Light Makes The Sky Blue

What color is refracted depends upon the critical angle, which is the angle the sunlight strikes
the back of the rain droplet. Red light bends the least, exiting the rain droplet at a 42 degree
angle, while Violet light bends the most, exiting the rain droplet at a 40 degree angle.

All of the other colors of the rainbow exit the rain droplets at some angle between 40 and 42 degrees,
thus making up the colors of the rainbow ROYGBIV (Red, Orange, Yellow, Green, Blue, Indigo and Violet), this order never changes.
Each rain droplet reflects all colors at a given point and time, but only one color comes back to your eye,
requiring million of rain droplets to create a rainbow. As the rain droplets fall through the sky, the
colors of the spectrum being reflected and refracted are constantly changing.

Rainbows form a complete circle, however only half is visible. The horizon only allows us to see half of the rainbow circle, so we see just an arc.
What causes the rainbow to have a circular formation is the way that rain droplets fall.

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source: Wikipedia/Concordia College