I love the XKCD webcomic, especially when it features a science-related theme. Comic number 1145 poses the question “Why isn’t the sky violet?” which I will attempt to answer this week. But first we need to understand why the sky is blue: it’s because of Rayleigh scattering.
Rayleigh scattering (named after the British scientist Lord Rayleigh) occurs when sunlight passes through the atmosphere and is scattered by air molecules. The light from the sun is a mixture of all the colors of the rainbow, each with its own characteristic wavelength. Sir Isaac Newton demonstrated this nearly 350 years ago by using a prism to separate white light into its different spectral colors.
|Rayleigh scattering: blue light is scattered |
more strongly than red light as it passes
through the atmosphere and is why the sky
is blue during the day.
Now back to the original question: since violet light has an even shorter wavelength than blue light, why does the sky appear blue instead of violet?
First, the sun produces a lot more blue light than violet light. The Sun’s spectral peak is in the green range and as the wavelength decreases from blue to violet there is a steep drop-off in intensity.
|Solar emission intensity compared to human cone cell responsivity. Both are shown |
as a function of wavelength.
|This image demonstrates how |
yellow light can be perceived
as a mixture of red and green
light. Take a few step back from
the monitor to see the effect.
It’s no coincidence that we see things the way we do. Human evolution is shaped by our environment—the ability to separate the colors around us provides an evolutionary advantage.
Even though humans don’t see violet in the sky, some birds might because they have an extra type of cone cell that extends their color vision into the ultraviolet range. The male Blue Grosbeak appears mostly blue to humans but has plumage shifted to the UV range that he uses to his advantage during courtship. The Common Kestrel uses its UV-enhanced vision to find voles by following their scent trail which reflects UV light, making it visible to this clever hunter. So maybe violet skies are for the birds.