Why Is The Sky Blue
“Because that blue sky that we all see is neither sky nor blue. What a shame that such beauty is not real!” These beautiful verses by Lupercio de Argensola are usually quoted as an example of the baroque disappointment for nature’s makeups. It was certainly during the 17th century when the man discovered with astonishment the reason for the color of the sky, that up to then had been believed to be compact and immutable.
The color theory developed by Newton explains that white light is formed by the combination of all the colors of the rainbow.
When we see an object of one color, it is because it reflects a certain wavelength and absorbs the rest, or which is the same thing, it reflects that color while absorbing the others.
At one end of the visible spectrum is red, whose wavelength is longer and therefore the lower frequency; because of this we call waves with a lower frequency “infrared”.
At the other end, violet, whose wavelength is shorter and therefore the highest frequency; therefore we call wavelengths with a higher frequency “ultraviolet”. But the sky, besides light composition, owes its color to another important factor: the atmosphere.
In effect, the light of the sun has to cross the atmosphere in order to reach us, and during that journey it looks altered. Minute particles of dust and water suspended in the air are smaller than the wavelengths of visible light, they do not have enough wave size to repel, but to divert slightly from its original path.
This is known as “dispersion”. And since the wavelengths on the blue end of the spectrum are shorter, they are dispersed to a greater degree than the rest of the colors, and this gives our sky its blue color.
To be exact, we need to add a third factor to determine why we see the sky as blue, and that is our own physiology. It happens so that, being consistent with what we have just said, the sky should be violet wavelengths as this color is still lower than in the blue.
If we see it violet is because our eyes capture the color in their own way, since they have cells sensible to only trhee colors: red, green and blue, and we perceive the rest of the colors by combination, that is, because they excite several of these three kinds of cells at the same time.
Therefore, as our sight is more sensible to the colour blue than to violet, it is that colour we observe when gazing at the sky.