Question

how can we explain the reddish appearance of sun at sunrise or sunset ? why does it not appear red at noon?

Answer 1

At the time of sunrise or sunset, the position of the sun is very far away from us. The sunlight travels longer distance through the atmosphere of the earth before reaching our eyes. Scattering of blue light is more than the scattering of red light. As a result of this, more red light reaches our eyes than any other colour. Hence sunset and sunrise appear red. 

During noon, the sun is overhead and sunlight travels less distance through the earth's atmosphere to reach our eyes. In this case, the scattering of almost all colours is very small. Hence, the sun appears white.

Answer 2

During sunrise or sunset, the light has to pass through greater distance in the atmosphere. The blue light is removed as it gets scattered the most while the red colour is less scattered and reaches the observer. Thus, we find reddish colour of the sun during sunrise or sunset.