Question

Why does the sun appear reddish at sunrise and sunset?
A. Near the horizon it passes through thick layers of atmosphere having larger particles
B. Sunlight travels longer distance at sunrise and sunset
C. Both A and B
D. None of the above

Answer 1

Hint: We have studied about the scattering of light. Many theories are given upon it but Rayleigh scattering theory is most appropriate. This question is solved by the concept of scattering of light which depends upon the wavelength of light.

Complete step by step answer:
According to the Rayleigh scattering theory, the intensity of the light at which the light is scattered depends upon the wavelength as: $I\propto \dfrac{1}{{{\lambda ^4}}}$. From this equation it is clear that the longer wavelength of light scattered less while the light which had a small wavelength scattered more.
When the sun rises or sets, it is just at the horizon and the light rays from the sun take more time to reach the earth so it has to travel longer through the atmosphere then the light has more interaction to the particles in its path. We know that shorter wavelengths are scattered more strongly than the longer wavelength of light. Blue colour has the shortest wavelength hence it is scattered more than the red colour and the red colour only reaches to the earth, hence the sun appears red at the sun set and sunrise.
However, light from the sun during the afternoon travels a relatively shorter distance. At noon, the Sun appears white as only a little of the blue colour is scattered whereas, near the horizon, most of the light of shorter wavelength at the blue end of the spectrum is scattered away by the small particles of the atmosphere.
Therefore, the light that reaches our eyes is of longer wavelength which gives rise to the reddish appearance of the sun.
Hence, A is the correct option.

Note:
Whenever we come up with this type of question, one must know that sunlight passes through thicker layers of air and larger distance in the earth’s atmosphere before reaching our eyes and most of the blue light and short wavelengths are scattered away by the particles.