How can we explain the reddish appearance of sun at sunrise or sunset? Why does it not appear red at noon?
Ans.
Amount of scattering is inversely proportional to fourth power of wavelength.
Wavelength of red colour is the largest so red colour is scattered the least by smoke or fog.
The reddish appearance of the sun at sunrise or sunset is because the wavelength of visible light from the sun near the horizon is greater than the scattering of light by the molecules of air and other fine particles in the atmosphere.
It passes through larger distance in the earth's atmosphere and thicker layers of air before reaching our eyes. Most of the blue light (shorter wavelengths) are scattered away by the particles.
So, only red light, being of highest wavelength reaches us which gives reddish appearance of sun at sunrise or sunset.
At noon the sun appears white because the light from the sun is directly overhead us. The light travels relatively shorter distance during the noon.
Scattering of light is reduced as the distance is reduced. So, the sun appears white as only a little of the blue and violet colours are scattered.