The science is clear: the sky, when it is clear of all clouds, is not blue. Why we see it this way is astonishing.
The blue of the sky is so much a part of the landscape that it would never occur to us to question its color. This is a reality shared throughout the world and can even create a sense of closeness between humans spread across the surface of the Earth. In the absence of science, early humans developed many beliefs by looking at the sky. Subsequently, the sky became a source of inspiration for artists of all genres, painters, poets and photographers alike.
But one of the questions that immediately comes to mind when we look up, from a very young age, is why is the sky blue? The explanation lies in the phenomenon of the diffusion of solar light in the atmosphere. Indeed, the color of the sky comes from the light of the Sun. Even though this light appears white to us, it is made up of all the colors of the rainbow. Light comes to Earth in the form of waves and each color has a specific wavelength. Some colors, like red and yellow, are longer than others, like blue and purple.
When light enters the atmosphere and collides with different molecules, notably oxygen and nitrogen, the light waves disperse, but not in the same way depending on the wavelength. Longer waves like red and orange scatter less while blue and violet scatter more.
However, it seems that this authoritative explanation is not entirely correct. Purple would in truth be the color that is most dispersed in the sky: the true color of the sky is therefore indeed purple! If the sky appears blue to us and not purple, this can be explained by the sensitivity of our eyes. Color perception depends on two types of photoreceptor cells present in the eye: cones and rods. The former are responsible for daytime vision and color perception while the latter are responsible for night vision and black and white. It is therefore the cones which come into play in the case of the color of the sky. There are three different types of cones, each more receptive to a type of wavelength and therefore color: red, green and blue. This means that the cones are more receptive to blue wavelengths than violet wavelengths and explains why we see the sky blue and not purple.
Color is also partly a subjective sensation created by the brain. The latter is capable of interpreting color based on context and sometimes even memory. This applies when a common object is under light that gives it a different color than usual. For example, even if the sky is pink/orange tones like at sunset, the brain knows that the sky is normally blue. However, in broad daylight, the brain does not have an example of a situation where the sky would not be blue and therefore cannot compare. Thus, the brain ignores the purple color of the sky to concentrate on the blue that it knows and which comes to it more easily thanks to the specificities of human vision.