Why is the sky blue?
The sunlight which illuminates the Earth is actually white. White light is nothing else but the combination of all colours (the entire spectrum of wavelenghts). When the object is exposed to white light, it absorbs almost all colours, but it still deflects some of them. That deflected part of the spectrum is the colour we see. This can only apply to objects that are intransparent, that don`t pass the light.
In the case of objects that are transparent, like glass, the situation is different. The white light reaches the glass, a small ammount is being deflected and the largest portion of the light keeps going through the object. Most of the colours which kept going on through the object have done so without any problems, but for certain photons in that group it is not as easy. Getting into the `Rush hour`, they start colliding with molecules and get spread around in all directions. The light of one color gets `spilled` all over the place and the glass becomes colored.
The same process is responsible for the color of the sky. The white light from the Sun reaches our planet and starts its journey through the atmosphere. The largest portion of the photons goes through undisturbed and reaches Earth`s surface, but some of them aren`t as lucky. According to the Rayleigh`s law the intensity of the scattered light is inversely proportional to the wavelength, meaning that the lower wavelength light will get scattered around much more than the higher wavelength light. If we remember that the highest value of wavelength belongs to the red light (about 700 nm), and the lowest to the blue (about 400 nm), we can easily conclude that the blue light will spread around the most. The light that reaches us does not contain the blue component because it gets spread all over the place, coloring the sky blue.
This process called Rayleigh scattering is also responsible for the sun color. It has already been said that the Sun emits the light of all colors, white light. When we look at it, we can see that it`s yellow, and it is yellow because the blue component `escaped` from the spectrum, giving the color to our planet Earth.
More information:
If you know a lot about the colors of lights and their wavelengths you may have gotten confused when you read that the lowest wavelength value belongs to the blue light. You were right! Color violet has the wavelength 380-450 nm. How is it then that the sky is not violet? Well, it is just too weak by the time it gets to the lower parts of atmosphere which we can see, because the higher parts have already absorbed the most of violet. Plus, or eyes are much less sensitive to the violet color. But still, some violet gets its way through and colors the sky. This can explain the `Sky blue` color.
Author: Zlatko Joncev
Translated and adapted by Uroš Todorović