Q. Rainbow is produced when sunlight falls on drops of rain. Which of the following physical phenomena are responsible for this?

1. Dispersion
2. Refraction
3. Internal reflection

Select the correct answer using the codes given below. (UPSC Prelims 2013)
Answer: 1, 2 and 3
Notes: The correct answer is [D] 1, 2 and 3. A rainbow is a natural spectrum appearing in the sky after a rain shower. It is caused by the combination of three distinct physical phenomena as sunlight interacts with spherical water droplets in the atmosphere.

• Refraction (Statement 2 is Correct): As sunlight enters a raindrop, it slows down and bends because it is moving from a rarer medium (air) to a denser medium (water). This initial bending is the first step in the process. Light is refracted again when it exits the droplet.
• Dispersion (Statement 1 is Correct): Sunlight consists of seven colors (VIBGYOR). Since different colors of light have different wavelengths, they bend at slightly different angles during refraction. This causes the white light to split into its constituent colors inside the droplet, a process known as dispersion.
• Internal Reflection (Statement 3 is Correct): After the light is dispersed into a spectrum, it hits the back surface of the raindrop. If the angle of incidence is large enough, the light undergoes reflection inside the drop (specifically, total internal reflection or partial internal reflection) and is sent back toward the front of the drop to eventually reach the observer's eye.

The formation of a primary rainbow involves two refractions (entry and exit), one internal reflection, and dispersion. Because all three phenomena are essential to separate the light and direct it back to the observer, all three statements are correct.