Did You Know the Sky is Not Blue?

Is the sky blue? If so, then why? Essentially, the way sunlight reacts with Earth's atmosphere and how our eyes interpret that light determines the blue sky we see. Light may disperse when it interacts with our atmosphere, much like when two pool balls meet and go off in various directions. Rayleigh scattering is the predominant kind of atmospheric scattering. A rough comparison would be to visualise photons scattering off molecules of air. 

The scattering rate is a function of the light's wavelength, or colour. Light scatters more when the wavelength is shorter. The shorter blue wavelengths are more widely diffused because the rainbow's colour spectrum, which runs from red to violet, reflects the transition between long and short light wavelengths. Because of all the scattered blue light, our sky seems blue. This is also the cause of the crimson appearance of sunsets. A sunset that seems reddish is left behind when blue light is dispersed.

But then why does the sky not appear violet? It's true that blue light scatters more than red or green light, but violet light should scatter more than blue because it has a shorter wavelength. The sky ought to be violet or at least violet blue. It turns out that although our eyes see blue, our sky is actually violet.

We cannot see certain wavelengths. Rather, three different kinds of colour-sensitive cells called cones are found in the retinas of human eyes. While the other two are most reactive to green and blue wavelengths, one type is highly sensitive to red wavelengths. The power of the signal from each type of cone, when light reaches our retina when we gaze at anything, gives our brains the ability to distinguish between the colours we perceive. 

There are slight variations between these colours and the real wavelengths we see. Cones may sense light of other colours, although their sensitivity is most at red, green, or blue. Blue cones are most stimulated by light with "blue" wavelengths, however red and green cones are also slightly stimulated. The sky would appear to be a somewhat greenish blue if the scattered light was, in fact, blue.

But as a result of the violet light from the sky, we are unable to perceive the greenish tint. The Earth's atmosphere scatters violet light the greatest, while our eyes' blue cones are less sensitive to it. Our red cones are slightly more sensitive to violet than our green cones, but they are not very good at perceiving blue or violet light. We would see violet light with a crimson hue if just violet wavelengths were scattered. However, the greenish blue and reddish violet tinges wash out when the blue and violet light from the sky are combined. The sky appears to be a light blue colour.