A rainbow is formed when sunlight is incident on water droplets suspended in the air due to

Analyze the question: We are asked what physical phenomenon is mainly responsible for the formation of a rainbow when sunlight meets water droplets.

Explain the concept:

• Refraction is the bending of light as it passes from one medium to another (air to water).
• Dispersion is the splitting of white light into its component colours. It happens because each colour has a different refractive index in the medium, so each bends by a different amount.
• Interference and diffraction involve the wave nature of light (superposition and spreading around obstacles). They are not the main cause of a rainbow.

Step-by-step solution:

1. White sunlight enters a raindrop and is refracted at the first surface.
2. Because each colour bends by a different amount (violet the most, red the least), the white light is separated into its colours. This is dispersion.
3. The separated light is totally internally reflected at the back of the drop.
4. It is refracted again as it leaves the drop, which spreads the colours further apart.
5. The observer sees the separated colours as a band of the spectrum: red, orange, yellow, green, blue, indigo, violet.

Refraction is involved, but it only bends the light. The thing that produces the separation into colours, which is what makes a rainbow, is dispersion.

Answer: C. dispersion

Tip: If a question asks about the splitting of white light into colours (rainbow, prism), think dispersion. If it asks only about bending of light (apparent depth, a stick looking bent in water), think refraction.