1.      How can we explain the reddish appearance of sun at sunrise or sunset? Why does it not appear red at noon?

Sun appears red at sunrise and sunset : At sunrise and sunset, sun is closer to the horizon. The sunlight passes through denser layer of air and covers larger distance before reaching our eyes. Most of the blue light gets scattered. And red color reaches us which make the sun appears red at sunrise.

During noon sunlight has to travel less distance to reach us. Because most of the colors get scattered we see sun white in color during noon.