Question

why is the sun red during sunrise and sunset

Answer 1

the SUN appears red during sunset and sunrise because at these times, sun is farthest from us and light has to travel the longest distance. particles in our atmosphere are as the same size as the light particles have. therefore, they scatter the light particles. so, the colours of light having highest wavelength like red, orange, yellow reach our eyes and others, due to their short wavelength scatter.
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Answer 2

Sun appears red at sunrise and sunset : At sinrise 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. The light that reaches our red. That is why the sun appears red at sunrise. 

At noon, the sun is overhead. The sunlight passes through rares layers of air and covers shorter distance before reaching our eye. So, almost all colours of light are scattered equally. Hence the sun appears white