# Why is the sky not violet?

If you have seen Veritasium’s latest video ‘The world in UV’, you would know that the sky gets it colour due to Rayleigh scattering. If you have not seen it yet, watch the relevant part below.

Since $I \propto \frac{1}{\lambda^4}$, light with shorter wavelength scatters more and dominates. This has been the reasoning for the very popular question ‘Why is the sky blue?’. And Derek takes it to the next level with a statement:

If we could see ultraviolet light, we might look up and ask, not why is the sky blue but, why is the sky ultraviolet‽

The complete version of Rayleigh scattering explains another interesting aspect also: Why the sun turns red during sunset?

But blue is not the least-wavelength-colour among visible light. There is violet. Thus appears the obvious question Why is the sky not violet?

And as on any nerdy-topic in this world, there is an xkcd comic on this:

There are 2 primary reasons for the sky to appear blue instead of violet:

## Emission spectrum of Sun:

Sun does not emit all frequencies equally. It acts like a blackbody at 5000K temperature. So, its emission peaks at yellow. Duh!

Observe the sharp drop-off on the left (violet side). Sun is just not hot enough to emits as much violet as blue!. But just this factor is not enough since we can still see some violet and indigo in the rainbows which also comes from the sun!

## Colour sensitivity of our eye:

The human vision works with Rods and Cones, millions of them. We have 3 types of cones responsible for sensing colour in the daytime. Each kind of cone individually observes blue, red and green light. All other colours in the universe will excite multiple cones at a time. For a human to perceive a white color, this means all the cones are stimulated. To see yellow, the green cone is stimulated a bit more than the red cone. If you see purple, then your red and blue cones are excited equally (and the green cone not very well). And, our cones are not equally sensitive to all colours.

Observe the dramatic drop off of blue curve for the lower wavelength (violet light). The human eye itself has a natural sensitivity or preference for blue light as opposed to violet light. The same is evident in the overall sensitivity as shown below:

So, unless we can see ultraviolet with good enough sensitivity, the question “Why is the sky ultraviolet?” will never really make sense.

## More interesting puzzles:

• What about the cameras? Don’t they sense all colours equally‽ If they do, why is the sky in its camera still blue and not violet?
• Normal sky is blue, sunsets are red. Why the sky passes from blue to orange/red skipping altogether the whole range of green frequencies?

If you find it interesting, feel free to discuss with me!