Blue colour of the sky is due to

The correct answer is A) Scattering of light. When sunlight enters the atmosphere, air molecules much smaller than the wavelength of visible light scatter it. This is Rayleigh scattering, and its intensity is inversely proportional to the fourth power of wavelength. Blue and violet are scattered far more than red, so the sky looks blue to us. Violet is scattered even more, but our eyes are less sensitive to it and the Sun's spectrum has less of it, so the sky appears blue.

• B) Dispersion: Splitting of white light into its colours because refractive index varies with wavelength, as in a prism or rainbow. It is not the reason for the sky's colour.
• C) Interference: Superposition of waves, seen in soap bubbles or Young's double slit experiment. It does not explain the colour of the sky.
• D) Emission: The Sun does not emit blue light in excess. Its output is roughly a near-blackbody spectrum peaking in the blue-green region, and the sky's colour comes from what the atmosphere does to that light.

This sits in the Physics section of General Science, and it links to the Mains question on atmospheric optics, including why sunsets look red and why heavy pollution can whiten the sky.

Trap: Candidates pick dispersion because it also produces colours, but dispersion separates colours in a medium and does not explain the sky.