Looking at the sky on a sunny day and seeing it tinted blue is something we often take for granted. But why is the sky blue? The answer lies in a physical phenomenon: the scattering of light.
Sunlight may appear white, but it is actually composed of all the colors of the rainbow. Each color of light has a different wavelength, ranging from violet (the shortest wavelength) to red (the longest wavelength).
On the horizon, sunlight is seen after passing through a large portion of the atmosphere, where the shorter wavelengths (like blue) have been mostly scattered, leaving primarily the longer wavelengths (red and orange).
Above, sunlight mainly scatters the shorter wavelengths (like blue), which gives the sky its blue color.
Image Wikimedia Rayleigh scattering
When sunlight enters the Earth's atmosphere, it interacts with air molecules, primarily nitrogen and oxygen. This interaction causes what is known as Rayleigh scattering.
This scattering phenomenon affects the colors of light differently, depending on their wavelength. Shorter wavelength colors (like blue and violet) are scattered in all directions much more than longer wavelength colors (like red and yellow).
Although violet light is scattered even more than blue light, our eyes are less sensitive to violet, and a lot of violet light is absorbed in the upper atmosphere. Therefore, blue light predominates and gives the sky its characteristic color.
Sunrise and sunset
You may have noticed that the sky takes on red and orange hues at sunrise and sunset. This is explained by the fact that when the Sun is low on the horizon, its light must travel through a thicker layer of the atmosphere. The shorter wavelengths (blue and violet) are scattered well before the light reaches our eyes, leaving the longer wavelengths (red and orange) to dominate.