Question

why does the sun appears red at sunset

Answer 1

During sunrise and sunset, the rays have to travel a larger part of the atmosphere because they are very close to the horizon.
Therefore, light other than red is mostly scattered away. Most of the red light, which is the least scattered, enters our eyes.
Hence, the sun and the sky appear red

Answer 2

hey,

***the distance of travel through the atmosphere increases during dawn and dusk, light is refracted much more than during noon. ... As the shorter wavelengths are scattered, only red light is left, as it has the largest wavelength of the seven colours. Thus the sun appears red during sunrise and sunset.