Why is the sky blue?

A question we’ve all asked as children, but sadly, as we grow we tend to stop wondering and start taking things for granted. Of course the sky is blue, what else would it be? Purple?

The truth is, the physics behind our blue sky is quite fascinating. It ultimately comes down to a concept called Rayleigh Scattering.

First, if you picture two waves, one with a high frequency and one with a low frequency, travelling the same distance between two points, you’ll notice the high frequency wave tends to “bounce up and down” more and therefore covers more distance in total as it travels.

This is depicted in the diagram: the blue wave has a high frequency while the red wave has a lower one.

Because the blue light covers a larger distance, it is more likely to hit particles, and be deflected, or ‘scattered’. When sunlight enters the atmosphere, the high frequency blue waves are more likely to hit the particles in the air and be scattered. This is why the sky is blue: it is the result of the scattering of blue light in the atmosphere.

In a sunset, light from the sun covers a larger distance as it approaches from a lower angle. This means the sunlight encounters even more particles, causing more blue light to deflect. The result is that the rest of the sky becomes an even deeper blue, while light coming directly from the sun appears to be red-tinted, because it lacks blue color.

So next time you look up at a sunset and gaze at its beauty, think of the reason why it looks that way… I guarantee it’ll make it seem even more mesmerising.