According to science, vision is the process of perception of electromagnetic waves in the light range by the human senses. Simply put, for an object to become visible, it must reflect the light coming from an external source or emit it itself. Only in this case will the characteristics of the object, such as contours and colour, be revealed.
In the case of the sky, the source of light is the sun. Sunlight is white, but the example of a rainbow shows that in the spectrum available to the human eye it has seven colours, which in turn depend on the wavelength of electromagnetic vibrations, with red being the longest and violet the shortest.
Looking from the ground to the sky, we see sunlight passing through the atmosphere, which is full of many obstacles on the path of the light wave. These are dust particles, gas molecules, ice crystals and water droplets – all these obstacles scatter the light flux, especially the short-wave (blue) part of it. Violet and ultraviolet waves, which are dangerous to human health, are largely absorbed in the upper atmosphere, so blue and blue shades become the most noticeable.
Why the sky is blue from the point of view of physics
Physics believes that white consists of only three colours, called primary colours:
- red
- green;
- blue.
This is due to the structure of the human eye: it has only three types of receptors that provide colour perception. When primary colours are mixed in different proportions, any colour in the visible part of the spectrum is obtained.
The structure of the rainbow was first described by Newton, who suggested that the sky has its colour because of the tiniest particles of water in the air. The weak point of this theory was the red sunrises and sunsets.
Two hundred years later, the English scientist Rayleigh, observing a ray of light that accidentally fell into a dark laboratory, asked a question: “If light propagates in a straight line, why is it visible from the side?”. He suggested that the light was reflected from small dust particles floating in the air. At that time, the electromagnetic nature of light was already known, and according to it, when a light ray encounters an obstacle on its way that is smaller than its wavelength, it is scattered. Obviously, in this case, the parts of the spectrum with the shortest wavelengths (blue and blue) will scatter most strongly. Therefore, the observer sees a clear sky as uniformly blue. Clouds are made up of water and ice, the particle size of which exceeds the wavelength of any spectral colour, so all colours are evenly scattered and the eye perceives the cloud as white.
Why the sky turns red at sunset
At sunset and before dawn, light falls to the ground at an angle, travelling a longer distance than in a direct hit when the sun is at its zenith. Waves of short length are scattered so strongly that they do not have time to reach the earth’s surface. In this case, the red rays are less scattered and give the viewer “burning” sunrises and sunsets.
By the way, astronauts who have visited the Moon have stated that due to the lack of atmosphere, the sky there is black.
