Question

Sky appears blue because of
A. reflection
B. Scattering
C. Refraction
D. Total internal reflection

Answer 1

Hint: Sky appears because the blue light enters our eyes from all the directions. This is why we see the colour of the whole sky as blue. Find the meaning of each of the options then analyse which one is the correct reason for this.

Complete step-by-step answer:
Sky appears blue because of scattering of light in the earth’s atmosphere. When light travels through a medium, small molecules (comparable to the wavelength of the wave) of the medium after absorbing incoming light radiations, emit them in all directions. This phenomenon is called scattering of light. When the electromagnetic waves of the sun travel through the earth’s atmosphere, the waves (radiations) are scattered. When the waves of the visible region (white light) get scattered in all directions, the sky is visible. But why does the sky appear to be blue, why not any other colour? It is because the intensity of scattered light depends on the wavelength of the light. Intensity of scattered light is inversely proportional to the fourth power of its wavelength ($\lambda$) i.e. $\text{intensity of scattered light }\propto {\displaystyle \frac{1}{{\lambda }^4}}$
Therefore, if the wavelength is longer, the scattering is less and if the wavelength is shorter, the scattering is more. That means the light with shorter wavelength is scattered more than the light with longer wavelength. Now, you may understand why the sky appears blue. Blue light has very small wavelength compared to the longer wavelengths in the visible region of the spectrum. Therefore, it is scattered the most and the sky appears to be blue.

Note: One may ask if blue light is scattered the most then why the sky appears to red at time of sunrise or sunset. At the time of sunrise or sunset, the rays of the sun enter the atmosphere from the direction of the horizon. Therefore, light has to travel a longer path in the atmosphere. Because of this, blue light is scattered continuously until there is no more blue light. Therefore, the scattered light that enters our eyes is of longer wavelength. Red has the longest wavelength and so the sky appears to be red.