Reason for blue colour of sky is explained by

The correct answer is Option (2) → Rayleigh Scattering

Sunlight is scattered by the air molecules, which are much smaller than the wavelength of visible light. For such small particles the intensity of the scattered light follows Rayleigh scattering:

    \( \displaystyle I_{\text{scattered}} \propto \frac{1}{\lambda^{4}} \)

So the shorter (blue) wavelengths are scattered much more strongly than the red ones, and the scattered light reaching the eye from all parts of the sky is mainly blue.

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 – “… 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.”