Diffusion of light in the atmosphere takes place due to

Diffusion (scattering) of light happens when sunlight strikes particles suspended in the air. The answer here is B, dust particles, because fine dust, smoke and aerosols scatter light in all directions. This is the same effect behind the Tyndall effect, the visible beam of sunlight through a window, and the haze that reduces visibility.

• A) Carbon dioxide: CO2 is a trace gas (around 420 ppm today) and its main atmospheric role is absorbing infrared radiation, which drives the greenhouse effect. It is not the cause of diffusion.
• C) Helium: Helium is an inert noble gas present only in negligible traces in the atmosphere. It plays no meaningful role in scattering.
• D) Water vapours: This is the close distractor, because clouds look white due to scattering by water droplets. But the question asks about diffusion in the atmosphere in general, and the standard textbook answer attributes it to dust particles. Vapour molecules as such are not the cause.

The core concept is scattering by particles comparable to or smaller than the wavelength of light. For Mains, link this to air pollution: PM2.5 aerosols reduce visibility, and aerosols also affect the Earth's radiation budget and climate, which is a recurring GS-III theme.

Trap: Aspirants often pick water vapour because of cloud whiteness, forgetting that the question is about the general atmospheric cause of diffusion.