Formation of Rainbows


When the Sun's light is refracted on passing through tiny rain droplets falling in the air a Rainbow is formed. Rainbows only need light from the sun or the moon and a large amount of spherical raindrops to form in the atmosphere. The rainbows can be compared to mini prisms. The light is refracted ("bending of light as it passes from one medium to another") and it causes different wavelengths (or colors) of the white light to separate. Whether these wavelengths of light will pass through the raindrop or reflect depends on the angle the light strikes the back of the raindrop. If a wavelength of the light strikes the back of the raindrop at an angle less than 48 degree relative to the normal then the light will pass through the raindrop. However, if the light strikes the back of the raindrop at an angle greater than 48 degrees then the light will be reflected. The reflected light is refracted as it exits the raindrop, but the color or the wavelength does not change. The following diagram shows how reflection causes the light to bounce off the raindrop.

Figure 1. Source:
http://redbaron.bishops.ntc.nf.ca/science/physics/papers/rainbow.htm


This process is repeated for a large number of raindrops, and a rainbow is formed. It's interestingto note, that the observer has his back to the sun and is facing the raindropswhen viewing a rainbow. Thus geometry needs to be just right between the observer,the place where it is raining, and the sun. The light from thesun passes over the observer, strikes rain droplets, and comes back to theobserver. Yet, the Earth prevents us to see the whole arc of a rainbow. Whenthe sun is higher in the sky, only a small arc of a rainbow cab be seen fromland. The reason for that is that the arc cannot be followed below the horizon(the droplets in the air below the horizon cannot be seen). It is possibleto see the full arc of rainbow when flying above the clouds in an airplane.