This phenomenon involves interaction of radiation with matter. Tiny dust particles are present in Earth’s atmosphere. When sunlight falls on them, it gets diffused in all directions. That is why light reaches even those nooks and corners where it normally is not able to reach straight from the source.
The phenomenon of scattering is a two step process: absorption of light by the scattering particle and then instant re-emission by it in all possible directions. The scattered light does not obey the laws of reflection.
The size of the particle must be less than the wavelength of light incident on it. A bigger sized particle will scatter all the wavelengths equally. The intensity of scattered light is given by Rayleigh’s law of scattering. According to this law, the intensity of scattered light is inversely proportional to the fourth power of its wavelength.
I ∝ 1/λ4
When white light is incident on the scattering particle, the blue light is scattered the most and the red light is scattered the least.
Blue Color of the Sky
The shorter wavelengths are scattered more than the longer wavelengths. Thus, the blue light is scattered almost six times more intensely than the red light as the wavelength of the blue light is roughly 0.7 times that of the red. The scattered light becomes rich in the shorter wavelengths of violet, blue and green colors. On further scattering, the violet light does not reach observe’s eye as the eye is comparatively less sensitive to violet than blue and other wavelengths in its neighbor. So, when you look at the sky far away from the sun, it appears blue.
White Color of the Clouds
The clouds are formed by the assembly of small water drops whose size becomes more than the average wavelength of the visible light (5000Å). These droplets scatter all the wavelengths with almost equal intensity. Therefore, the resultant scattered light is white. So, a thin layer of clouds appears white.
Red Color of the Sun at Sunrise and Sunset
In the morning and evening when the Sun is near the horizon, light has to travel a greater distance through the atmosphere. The violet and blue wavelengths are scattered by dust particles and air molecules at an angle of about 90°. Thus, the sunlight becomes devoid of shorter wavelengths and the longer wavelength of red color reaches the observer. So the Sun appears as red.