Why is the sky blue?

(via MentalFloss)

Turns out, it isn’t.  That’s the short answer.  The long answer is interesting – it’s violet, but our daylight vision is such that it is seen as blue.

Why is the sky blue? It is a question children ask. Yet it also intrigued Leonardo da Vinci and Isaac Newton, among many other legendary thinkers. As late as 1862, the great astronomer John Herschel called the colour and polarization of skylight “great standing enigmas.” Even today, our perception of sky blue is little understood by laymen.

. . .

In 1873, James Clerk Maxwell realized the truth: The scattering substance must be the molecules of air themselves, none other. Indeed, John Ruskin suggested in 1869 that sky blue comes from light “reflected from the divided air itself.” Lord Rayleigh followed Maxwell’s advice and calculated that the observed scattering of skylight requires molecules whose size accords with that indicated by other physical arguments.

. . .

As you gaze at the clear blue sky, then, you are beholding unambiguous evidence that atoms really exist, something that was widely questioned as recently as the 19th century. This presents a profound but simple answer to our opening question about the sky’s colour. But even so, further explanation is required.

If Tyndall and Rayleigh are right, then the violet wavelengths from the sun, having still shorter wavelengths than blue, should be scattered even more. Given this, shouldn’t the sky be violet, not blue?

Indeed the sky is violet, if you observe not with the naked eye but with an instrument that objectively measures the intensity of the spectrum at different wavelengths. Such a device, a spectrophotometer, shows that, in fact, the highest peak of the intensity of skylight occurs in the violet range.

But why do we see blue, nonetheless? The resolution of the mystery lies in our daytime vision, which happens to be eight times less sensitive to violet than to blue light.

Does that mean it is “incorrect” to call the sky blue? Not really. Our names for colors reflect our common perception, whatever a mechanical instrument might say.

There you have it.  The sky is violet.  But we still call it blue.  Our eyes don’t see violet as well as blue, and the higher intensity of violet is not sufficient to overcome our innate sensitivity to blue.  That means more blue is registered by the cones in our eyes, so our brain just gives us what it has more of to work with.

[tags]Why is the sky blue, Eye color sensitivity[/tags]