CAPF AC 2022 Q23: Science & Technology – Physics

CAPF AC 2022, Paper I (General Ability and Intelligence) · Science & Technology

1. (a) dispersion
2. (b) reflection
3. (c) refraction
4. (d) scattering

Answer: (d) scattering

Explanation

Why the sky is blue

How to think: the sky has no colour of its own.

Sunlight hits the tiny molecules of air and is thrown in all directions.

This is scattering.

Blue light, with its short wavelength, is scattered far more than red.

The rule: in Rayleigh scattering the amount scattered is proportional to 1/λ⁴.

So shorter wavelengths scatter much more.

Without an atmosphere: there is nothing to scatter light, so the sky looks black, as astronauts see it.

Answer: (d) scattering.

Why each option is right or wrong

• (a) Incorrect. Dispersion splits white light into colours, as in a rainbow.
• (b) Incorrect. Reflection sends light back from a surface; it does not colour the sky.
• (c) Incorrect. Refraction bends light; it explains twinkling stars, not the blue sky.
• (d) CORRECT. Scattering of sunlight by air molecules makes the sky look blue.

Key point

Rayleigh scattering ∝ 1/λ⁴: blue scatters most → blue sky. At sunrise and sunset light travels farther, blue is scattered away → red sun. Danger signals are red because red is scattered least. Clouds look white due to scattering by large drops.

Remember

Blue sky and red sunset: scattering.

Common mistakes

• Marking dispersion because it also involves colours.
• Thinking the sky is blue because it reflects the sea.
• Forgetting that the same scattering makes the sunset red.

Exam tip

Educated guess: NCERT's chapter heading is scattering of light, and its first example is the blue colour of the clear sky.

Sources

• NCERT Class X Science — The Human Eye and the Colourful World (scattering of light)
• NCERT Class XII Physics — Ray Optics and Optical Instruments (some natural phenomena due to sunlight)

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