The phenomenon responsible for the blue colour of the sky

The correct answer is Option (3) → Scattering of light

Sunlight is scattered by the air molecules, which are much smaller than the wavelength of light. By Rayleigh's law the scattered intensity varies as \( 1/\lambda^{4} \), so the short blue wavelengths are scattered far more than the red ones, and the scattered blue light reaches the eye from every direction of the sky.

Option (3) Scattering of light — Correct. Scattering by air molecules favours short wavelengths, which gives the blue sky.

Note: This question is from the pre-rationalised NCERT book (2022-23). The topic was removed from the latest NCERT book (2026-27), and it also appears to be out of the CUET 2026 syllabus.

NCERT (Pre-rationalised edition, 2022-23):
Scattering of Light – “As sunlight travels through the earth’s atmosphere, it gets scattered (changes its direction) by the atmospheric particles. Light of shorter wavelengths is scattered much more than light of longer wavelengths. (The amount of scattering is inversely proportional to the fourth power of the wavelength. This is known as Rayleigh scattering). Hence, the bluish colour predominates in a clear sky, since blue has a shorter wavelength than red and is scattered much more strongly.”