Question

Rainbow is cause due to:
(A) Reflection of sunlight
(B) Dispersion of sunlight from water drops
(C) Refraction of sunlight from water drops
(D) Refraction of sun rays by air molecules

Answer 1

Hint : Dispersion is the splitting of a non monochromatic light wave as it crosses the boundary between two media. Sunlight is a non monochromatic light with seven different light waves which have different wavelengths. Rainbow is visible only on rainy day.

Complete step by step answer
When light is emitted from the sun, and enters the atmosphere of the earth, mostly, on a clear day, the sunlight goes through the atmosphere almost un-refracted or dispersed through the air, since air only has an extremely low refractive index, hence negligible under most circumstances.
However, on a rainy day, water droplets with significant size are present in the air. When light waves from the sun are incident on these water droplets a phenomenon known as dispersion occurs. Dispersion is the splitting of a non monochromatic light wave as it goes from one medium to another of different refractive indices. Dispersion is caused by refraction of the constituent light waves. Different wavelengths of light refract differently when they cross the boundary of a medium. Some colours of light waves are refracted at a larger angle than others, hence, allowing a distinction of the different colours making up the wave.
Hence, the correct answer is B and C.

Note
For clarity, different wavelengths refract differently because they travel at different speeds in a material medium. Since, the refractive index of a material is slightly different for each wavelength of light, making them have a different refraction angle when they cross the boundary of two media.