Dispersion And Scattering

NCERT explains the blue sky through Rayleigh scattering, in which the intensity of light scattered by tiny air molecules is inversely proportional to the fourth power of the wavelength, $I\propto \frac{1}{{\lambda }^4}$. Because blue light has a shorter wavelength than red, it is scattered far more strongly, roughly by a factor of several times, so scattered blue light reaches the eye from every part of the sky. This is why the daytime sky glows blue while the direct sun, with its blue removed, looks slightly yellowish. The option that blue travels faster is wrong because scattering, not speed differences, governs sky colour, and all colours travel at essentially the same speed in air. The option about total absorption is wrong because the atmosphere is largely transparent to visible light, not selectively absorbing. The option about water vapour reflection is wrong because the effect persists even in dry clear air, so molecular scattering, not vapour, is responsible. As stated in NCERT Class 12, Chapter 9, the strong wavelength dependence of Rayleigh scattering explains both the blue sky and red sunsets. A consistency check confirms that at sunset the long path scatters away blue, leaving red, which agrees with the same law.