When light passes from one medium to another, say air, a glass of water, then a part of the light is absorbed by particles of the medium, preceded by its subsequent radiation in a particular direction. This phenomenon is termed a scattering of light.
The color of the sky during the daytime is blue because the molecules in the air scatter blue light more than the red light because the wavelength of the blue light is shorter than the other light.
Rayleigh's scattering theory is reasoned for the red color of the sun in the morning and the blue color of the sky.
Let be considered as the probability of scattering and is the wavelength of radiation, then it is given as:
3. Thus, the probability of scattering will increase for a shorter wavelength as it is inversely proportional to the fourth power of the radiation wavelength.
4. Since blue does indeed have a shorter wavelength whereas red has different wavelengths (longer), red is capable of reaching our eyes following air dispersion of light.
5. At sunrise and sunset, the Sun is very low in the sky, which means that the sunlight we see has traveled through a much thicker amount of atmosphere. Because blue light is scattered more strongly by the atmosphere, it tends to be scattered several times and deflected away in other directions before it gets to us. This means that there is relatively more yellow and red light left for us to see.
6. The diagram below shows that at midday, light has less atmosphere to travel through. Reaching Earth's atmosphere, it is scattered in all directions, therefore we see blue sunlight. Blue is scattered more than other colors because it travels as shorter, smaller waves. However, at sunset, light has further to travel through the atmosphere. The shorter wavelength of blue light is scattered further, as the sunlight passes over a greater distance, and we see the longer wavelength of yellow and red light.
Hence, during sunrise and sunset, the sky appears red.
