The sun looks reddish-orange during sunrise and sunset. What is the reason behind it?

The sun looks reddish-orange in the morning and the
evening due to a phenomenon call Rayleigh scattering. In the morning and evening the
sunlight has to travel the longest distance through air. The molecules of nitrogen and
oxygen which are the primary constituents of air scatter the wavelengths that are close
to their size. These are the wavelengths towards the violet end of the
spectrum.

Light waves that lie towards the red end of the
spectrum are not scattered as much. This allows them to travel through the atmosphere
relatively unhindered. By the time the light from the Sun reaches the observer a large
part of the blue and violet components of the light has been absorbed and the red and
orange components remain.

This makes the sun appear
reddish-orange in the morning and evening.