UPSC Prelims 2003 Q31: Science & Technology – Physics
Diffusion of light in the atmosphere takes place due to
- (a) Carbon dioxide
- (b) Dust particles
- (c) Helium
- (d) Water vapours
Answer: (b) Dust particles
Explanation
Diffusion of light in the atmosphere
| Option | Role |
|---|---|
| Dust particles | Yes — scatter and diffuse sunlight |
| Carbon dioxide | Absorbs infrared, not a diffuser of visible light |
| Helium | Negligible in the atmosphere |
| Water vapour | Absorbs infrared |
Diffusion (scattering) of light in the atmosphere takes place mainly due to dust particles (along with air molecules), which is why the sky is bright even away from the Sun.
Answer: (b) Dust particles.
Why each option is right or wrong
- (a) Incorrect. Carbon dioxide absorbs infrared radiation.
- (b) CORRECT. Dust particles scatter and diffuse light.
- (c) Incorrect. Helium is negligible in the air.
- (d) Incorrect. Water vapour mainly absorbs infrared.
Key point
Scattering: Rayleigh by molecules, Mie by dust; red sunsets from dust and longer paths.
Remember
Dust particles diffuse sunlight in the atmosphere.
Common mistakes
- Choosing water vapour because of clouds.
- Confusing scattering with absorption.
- Thinking that only gases scatter light.
Exam tip
Dust: diffuse reflection; molecules: blue sky.
Sources
- NCERT Class XII Physics — Ray Optics
- NCERT Class X Science — The Human Eye and the Colourful World (scattering of light)