Question

why sky is red during sunset?

Answer 1

Within the visible range of light, redlight waves are scattered the least by atmospheric gas molecules. So at sunrise and sunset, when the sunlight travels a long path through the atmosphere to reach our eyes, the blue light has been mostly removed, leaving mostly red and yellow light remaining

Answer 2

it is due to the atmospheric refraction of light.

during sunset, the sun is far away from the earth and when the sunlight comes and passes through the atmosphere, the light splits into its component colours. other colours are absorbed by the particles and the red colour get spreaded in the atmosphere.