Which of the following phenomena contributes significantly to the reddish appearance of the sun at sunrise or sunset?
(a) Dispersion of light
(b) Scattering of light
(c) Total internal reflection of light
(d) Reflection of light from the earth
Option (b) is correct.
The red colour in the sky at sunset (and sunrise) is due to an effect called Rayleigh scattering. There is a similar form of scattering called Mie scattering which is responsible for the white colour of clouds.
Particles in our atmosphere that are approximately of the same size as that of the wavelength of visible light because the white light from the sun to scatter and split into individual components. Oxygen and Nitrogen (the main components of our atmosphere) scatter violet and blue light due to their small size. This is why the sky appears to be blue in the daytime, especially at midday when the Sun is closest to us.
During sunrise and sunset, the distance that the light has to travel from the Sun to an observer is at its greatest. This means that a large amount of blue and violet light has been scattered so the light that is received by an observer is mostly of a longer wavelength and therefore appears to be red.