Question

Why does the clear sky appear blue?

Answer 1

Hint: The colour detected by our eyes is basically the wavelength of the light received by us through the retina of our eye. When the sunlight passes through the atmosphere it undergoes elastic scattering due to the molecules present in the atmosphere. The intensity of light received is more for light of minimum wavelength. Hence we can come to a conclusion that light of minimum wavelength will be seen.

Complete step by step answer:
Now let us understand on what basis the scattering of light is governed when it passes through the atmosphere.
As the sunlight passes through the layers of atmosphere during the day molecules of nitrogen and oxygen absorb the sunlight and reemit it. The mixture of gas scatters light in all the directions. This scattering of light is elastic scattering since there is no absorption of energy. Hence it obeys Rayleigh’s law of scattering which is mathematically given as $I\propto \dfrac{1}{{{\lambda }^{4}}}$ where I is the intensity of the scattered light and $\lambda $ is the wavelength of the light scattered. Since among all the colors of light, blue has a minimum wavelength intensity of light scattered and hence perceived by our eyes is blue. Hence the clear sky appears blue.

Note:
If the earth had no atmosphere, there would have been no scattering and the sky would have appeared black and we could have seen the stars during the day time. This is what astronauts actually observe when they are about to exit the earth’s atmosphere. Sometimes during evenings the sky appears red this is because the sun as it reaches the horizon, it passes through more layers of atmosphere and the intensity of blue light completely gets scattered and we see the light of maximum wavelength i.e. red.