At sunrise and sunset the rays from the sun have to travel a larger part of the atmosphere than at noon. Therefore most of the blue light is scattered away and only the red light which is least scattered reaches the observer. Hence, sun appears reddish at sunrise and sunset
The atmosphere contains free molecules and atoms with different sizes. this molecules and atoms scatter light of different wave lengths which  are comprable to their size. molecules having a size that is comprable to the wavelength of red light are less in the atmosphere. hence scattering of red light is less when compared to the other colours of light.the light from the sun needs to travel more distance in atmosphere during sunrise and sunset to reach your eye. in morning and evening times during sunrise or sunset, except red light ,all colours scatter more and vanish before they reach you. since scattering red light is very  small, it reaches you.As a result sun appears reddish in sunset and sunrise.