Formation of Rainbow

Rainbow is a beautiful natural phenomenon that occurs when sunlight is refracted, dispersed, and reflected by water droplets in the atmosphere. The process of rainbow formation involves two main optical phenomena: dispersion and total internal reflection.

Dispersion:
When white sunlight passes through a raindrop, it is refracted (bent) as it enters the droplet, and then it undergoes dispersion. Dispersion is the phenomenon where different colors of light have different refractive indices, causing them to bend at different angles. This is due to the fact that the speed of light varies with its wavelength. The shorter wavelength light (violet and blue) bends more than the longer wavelength light (orange and red).

Total Internal Reflection:
After undergoing dispersion, the light rays inside the droplet are reflected off the inner surface of the droplet. This reflection occurs because the angle of incidence is greater than the critical angle for total internal reflection. As a result, the light rays bounce off the surface and are directed back into the droplet.

Emergence of Rainbow:
The light rays that are reflected inside the droplet undergo multiple internal reflections before finally exiting the droplet. As they exit, they are again refracted and dispersed. The different colors of light are spread out and form a circular arc in the sky. This circular arc is known as a primary rainbow.

Primary and Secondary Rainbow:
The primary rainbow is characterized by the inner edge being violet and the outer edge being red. The secondary rainbow is formed by a similar process, but with an additional reflection inside the raindrop. The secondary rainbow appears above the primary rainbow and has reversed colors, with the inner edge being red and the outer edge being violet.

Conclusion:
In conclusion, the formation of a rainbow is a result of the combination of dispersion and total internal reflection. Dispersion separates the sunlight into its constituent colors, and total internal reflection causes the light to bounce off the inner surface of the water droplets. This combination creates the beautiful phenomenon that we observe as a rainbow in the sky.