The colours of the observed sky change depending on the time of day. In cloudless conditions, in the afternoon hours, an intense blue is visible, but at sunrise and sunset, yellow, orange and red colours appear. The colours we see in the sky are the result of an optical phenomenon called Rayleigh scattering.
Light as an electromagnetic wave
The light coming from the Sun is nothing but an electromagnetic wave. Electromagnetic radiation is one of the ways the energy is propagated through space. If electromagnetic waves are ordered according to their characteristic parameters, e.g. frequency, length, etc., we obtain a spectrum. The electromagnetic spectrum represents all types of radiation that exist in the universe.
Visible light (range from 380 nm to 780 nm) is only a small part of the entire electromagnetic spectrum, namely the part that we are able to see. It behaves both like a wave, and as a stream of particles. This phenomenon is called wave-particle duality. White light is created by combining several simple colours, called primary colours. They are visible, for example, as a rainbow (white light dispersed in water droplets). Within the electromagnetic spectrum corresponding to white light, red light has the longest wavelength, and violet light has the shortest.
Why do we see specific colours?
Each range of the white light spectrum has specific colours assigned. Such radiation falling on a particular object will be absorbed or reflected by it. For example, a leaf that receives light absorbs it, with the exception of the green wavelength. The unabsorbed fragment of the spectrum is scattered in all directions. Some of this radiation will reach the eye and the brain will “tell” that the leaf is green. We observe other colours in a similar way, e.g. strawberry red. Read more here.
Rayleigh scattering
Looking at the sky, we are actually observing the part of the atmosphere that is visible from the surface of the Earth. The light wave coming from the Sun does not travel in a straight line. It encounters a number of obstacles on its way. These are many molecules (mainly nitrogen and oxygen), dust, water droplets and ice crystals suspended in the air. They cause scattering of visible light, its reflection or absorption. In order for radiation to reach Earth, it must pass through all these obstacles. It turns out that when it falls on particles suspended in the atmosphere, the light is scattered, and from the entire spectrum, the blue colour is the most affected. This effect is called Rayleigh scattering and is responsible for the blue colour of the sky.
In 1899, John Rayleigh proved that the intensity of scattered light is inversely proportional to the fourth power of the wavelength. This means that blue light (which we see in the sky) is four times more scattered than red light because it has a shorter wavelength in the white light spectrum. However, the wavelengths corresponding to violet colour are shorter, so the sky should be purple. Why isn’t it so? One of the reasons for this phenomenon is that the intensity of violet radiation that reaches the Earth is much lower. In addition, our eyes are several hundred times less sensitive to violet light compared to blue.
Other colours of the sky
The way we see the sky is caused by the phenomenon of Rayleigh scattering, which states that short wavelengths, responsible for the blue colour, are much more strongly scattered than long wavelengths. However, the colour of the sky changes during the day. The blue sky is only observed for a few hours in the afternoon. Why does it change at sunrise and sunset?
Why is it that when the sun goes down, the colour of the sky changes from blue, through yellow, and finally to intense red? This is related to the path that the sun’s rays must travel to reach the Earth. The lower the Sun shines in the heavenly vault, the greater this distance. White light, therefore, falls at a small angle and shorter wavelengths, i.e. those assigned to violet and blue colours, are scattered too much and do not reach the eye of the observer. Only the colours characteristic of long wavelengths, i.e. yellow and red, are visible. They cause the characteristic colour of the sky at sunset.
As a curiosity, it is worth mentioning that the formula proposed by Rayleigh refers to scattering on small particles suspended in the atmosphere. It does not relate to e.g. water drops or ice crystals. Scattering on larger species is explained by Mie solution, described in 1908. It says that white light falling on, for example, clouds, which consist of water and ice particles, is scattered equally, regardless of the wavelength. As a result, the clouds appear white.
Read also: how do we describe colours?