Q.Explain the formation of a rainbow.

Sunlight refracts on entering a raindrop (dispersing into colours), totally internally reflects once off the back of the drop, and refracts again on exiting - each colour leaves at a slightly different angle, spreading the colours into the arc we see as a rainbow.

Explanation

A rainbow is a natural example of dispersion combined with refraction and total internal reflection (TIR), seen when sunlight falls on tiny spherical water droplets suspended in the air (typically just after rain, with the Sun behind the observer).

Path of light through a single raindrop (primary rainbow):

1. Sunlight (a mixture of all colours) enters a spherical raindrop and is refracted at the front (curved) surface. Since the refractive index of water is different for each wavelength, the different colours are refracted by different amounts - this is dispersion, and it begins separating the colours slightly.
2. The refracted, now slightly-dispersed light travels to the back surface of the drop, where it undergoes total internal reflection (the angle of incidence there exceeds the critical angle), reflecting the light back into the drop.
3. The reflected light then refracts again as it exits the front surface of the drop, dispersing the colours further. …