$(a)$ Explain why the colour of a clear sky appears blue during the day with the help of a diagram.
$(b)$ Explain why the Sun looks reddish at the time of sunrise and sunset.

(N/A) The Earth's atmosphere contains air molecules and other fine particles. These particles are smaller in size than the wavelength of visible light. According to Rayleigh scattering, the amount of scattering is inversely proportional to the fourth power of the wavelength $(I \propto 1/\lambda^4)$. Since blue light has a shorter wavelength compared to red light, it is scattered much more strongly by the atmospheric particles in all directions. When this scattered blue light enters our eyes, the sky appears blue.
$(b)$ At the time of sunrise or sunset, the Sun is near the horizon. Sunlight has to travel through a much thicker layer of the Earth's atmosphere to reach the observer. During this long journey, most of the blue and shorter wavelength light is scattered away by atmospheric particles. Only the light with the longest wavelength, which is red light, is able to reach the observer's eyes without being scattered significantly. Therefore, the Sun appears reddish at sunrise and sunset.