Question

Bengali essay on the colour of the sky during sunset and sunrise

Answer 1

The issue is you are seeing a combination of two different types of scattering. The first is Rayleigh Scattering. This is scattering off Oxygen and Nitrogen molecules that make up the air itself.

These tend to scatter blue light much more strongly than red, orange, and yellow light. This is what makes the sky appear blue. And when the sun is nearly at the horizon the length of atmosphere that

the light must pass through to reach your eye

becomes about 5 times as long, meaning five times

more blue light is scattered when the sun reaches

the horizon as when it is overhead.

Rayleigh scattering therefore removes blue light, and a lot of green, and even some yellow light, and what is left is mainly reddish/orangish light.

That's the first part of the answer. But then you also have scattering due to aerosols, and this can include dust, but also haze (including sea spray, etc.). Now, these particles do not scatter light at all like

Rayleigh scattering. About half of the light scattered by these particles is in the forward direction. That is,

when you're looking toward the sun you're seeing

the light that's coming directly from the sun plus

light that's been scattered and appears to be coming from the region around the sun. This makes the whole sky in the direction of the sun appear more

reddish/orangish. It also produces illumination

effects on clouds and mountains that makes them

appear reddish/orangish on the side facing the sun.

You'll also see that the back sides of these clouds

appear more bluish because that extra blue light that was scattered has not disappeared, it's merely flowing in different directions, including onto the backs of the clouds.