The phenomenon used for explaining the formation of a rainbow in the sky is

Analyze the question: It asks for the phenomenon that explains how a rainbow forms. Several processes are involved, but we need the one that explains the colours appearing in the sky.

Concept: A rainbow forms when sunlight (white light) enters raindrops, and three things happen:

1. Refraction: light bends as it enters the water droplet.
2. Dispersion: white light splits into its component colours (red, orange, yellow, green, blue, indigo, violet). This happens because each colour has a different wavelength and therefore a different refractive index in water. Violet bends the most and red the least.
3. Internal reflection: the light reflects off the back of the droplet.
4. Refraction again: the light bends once more as it leaves the droplet, and the separated colours reach the observer's eye.

Solution:

• Interference (A) involves the superposition of waves, as in thin soap films. It is not the main cause of a rainbow.
• Refraction (B) is part of the process, but refraction alone only bends the light. It does not explain why the colours separate.
• Reflection (C) occurs inside the droplet, but it does not separate colours.
• Dispersion (D) is the splitting of white light into its spectrum of colours. This is what gives a rainbow its band of colours.

Answer: D. dispersion

Tip: Think "rainbow = colours separated = dispersion." If a question asks about the separation of white light into colours (rainbow, prism), the answer is dispersion. If it asks about the bending of light as it changes medium, the answer is refraction.