This question comes from The question every child asks, and every parent dreads.
To answer this, we first must answer: What is Color? We previously covered this so if you haven’t read that yet click the link above:
White light comes into our atmosphere from the sun and gets scattered by the oxygen and nitrogen molecules within it. But the interesting thing is that the don’t get scattered uniformly. Smaller wavelengths get scattered with higher preference than longer wavelengths.
This is a rough graph showing how much scattering is occurring at each wavelength and color. As you go down in wavelength into blue and violet, more scattering occurs.
So, if you’re looking up at the sky, the lower wavelengths scatter at a higher rate, and therefore we’re going to see those colors.
Now you may be wondering, if the lower wavelengths scatter more uniformly, then why isn’t the sky violet or purple?
Technically it is, but we can’t see it. Our eyes perceive color using rods and cones which are sensitive to different wavelengths. The cones are used for sensing bright light and the rods are used in low light. And we have three different types of cones in our eyes sensitive to different ranges of wavelengths of light. Below is a graph of the sensitivities for the different cones at different wavelengths:
As a result, even though our atmosphere is scattering more violet wavelengths, our eyes are more sensitive to the blue wavelengths. This in combination with the mixing of all the other colors being scattered at lower amounts makes the light we see equivalent to a pure blue light mixed with white light, which we see as blue instead of the violet light it is in reality.
Other animals that have different cones would likely see the sky as a different color. Bees have been studied and shown to have receptors that can see into the wavelengths below our visible range so to them the sky is likely a color we cannot even perceive.
So, the moral of the story is: don’t get into an argument with a bee about the color of the sky.
