" The Sun and the surrounding sky appear red at sunrise and at sunset because at that time most of the blue colour present in sunlight has been scattered out and away from our line of sight, leaving behind mainly red colour in the direct sunlight beam that reaches our eyes".
This can be explained as follows.
At the time sunrise and sunset when the sun is near the horizon, the sunlight has to travel the greatest distance through the atmosphere to reach us. Durning this long journey of sunlight, most of the shorter wavelength blue-colour present in it scattered out and away from our line of sight. So, the light reaching us directly from the rising sun or setting sun consists mainly of longer wavelength red colour due to which the sun appears red.
Due to the same reason, the sky surroundings the rising sun and setting sun also sun also appears red. Thus, at sunrise and sunset, the sun itself as well as the surrounding sky appear red.
We will now discuss why the sun appears white it is overhead in the sky. When the sun is overhead (as at noon), then the light coming from the sun has to travel a relatively shorter distance through the atmosphere to reach us.
During this shorter journey of sunlight, only a little of the blue colour of the white light is scattered (most of the blue light remains in it). Since the light coming from the overhead sun has almost all it's component colour in the right proportion, therefore, the sun in the sky overhead appears white to us.
Experiment to study Scattering of light
We will now perform an experiment to understand how the scattering of light leads to the blue color of the sky, and the red appearance of the sun at sunrise and sunset. In this experiment we will prepare a colloidal solution containing tiny particles of sulphur required for scattering the light by the action of sulphuric acid on sodium thiosulphate solution.
Set up appears as shown in figure. S is a strong source of white light. We will consider this source of light to be the sun . The source of light S is placed at the forcus of a convex lens (converging lens) L1 so as to produce a parallel beam of light rays.
A transparent glass tank T is filled with about 2 litres of clear water. A cardboard disc D having a circular hole C at its centre is kept on the other side of the water tank. Another convex lens L2 is kept behind the cardboard disc to forcus the light rays (coming from the glass tank and passing through the circular hole of cardboard disc) to from an image the screen R. We will now describe how the experiment is actually performed.
Switch on the source of light S. We will find a beam of light passes through water in the glass tank and forms a circular patch of white light on the screen R. We can, however, not see the path of the beam of light inside the water of the tank (because there are no suspended particles in water to scatter the beam of ligh). Let us now dissolved about 200 grams of sodium thiosulfate (called 'hypo') in water of the glass tank.
Then add 1 to 2 mL of concerted sulpuric acid to the water. We will see that fine microscopic particles of sulphur begin to form in water and a colloidal solution is obtained. Ad the sulphur particles begin to form in water, we will see the blue light coming from the sides of the glass tank. This is due to the scattering of shot wavelength blue light (present in the beam of white light) by the minute colloidal sulphur particles. This is how the sky blue (, when the blue light present in sunlight is scattered by the molecules of air in the atmosphere).
If we look at the screen on the front side of glass tank containing coloidals solution of sulphur, we will see a red patch on the screen. This is because mainly the red colour of the beam of white light reach th screen after passing through the colloidal sulphur solution in the glass tank (the blue colour being scattered away and hence eliminated on the way).
This is how the sun looks red at sunrise and sunset when mainly the red color of sunlight reaches our eyes after the Eli of blue colour through scattering along the away.
