David Boettcher - Professional Engineer

Eur Ing D B Boettcher BSc(Hons) CEng MIET

Providing innovative solutions to engineering and business ~~problems~~ opportunities.

Why Is The Sky Blue?

Over the years I encountered various explanations about why the sky is blue; but none of them really explained it, instead relying on vague allusions to "scattering". I decided to find out more, and this is my simple take on why the sky is blue; it helped me to understand it, I hope it might help you.

Scattering of blue light makes the sky look blue

The are two fundamental things one has to bear in mind when thinking about why the sky is blue.

The first is that all light that enters your eyes, unless it comes direct from a light source, arrives after bouncing off something. So if you are outdoors on a nice sunny day, unless you look directly at the sun (which you should never do) then all the light arriving at your eyes has bounced off something, such as leaves of grass, a tree, or a dog, or whatever it is that you see.
The second is that without our atmosphere the daytime sky would appear black. The sky wouldn't actually be black, it wouldn't be any colour at all; you would just see directly out into space, like astronauts do from the surface of the moon. Or as we do at night. Without the atmosphere, any sunlight that didn't hit something on the surface of the earth would just stream past unseen.

Bearing in mind these two fundamental things, it is obvious that the atmosphere is doing something that changes or directs some of the light from the sun that would otherwise just flash past unseen into a blue light that enters our eyes. This is where the scattering comes in.

As light from the sun enters the atmosphere, some of it hits the oxygen and nitrogen molecules and is scattered in all directions. This is called Rayleigh scattering, after the British physicist Lord Rayleigh who first explained it. Some of the scattered light comes down towards you, and it is this light that you see as the colour of the sky. But why is it blue?

The sun gives off light in all colours of the spectrum. We see this as white, but we know from the studies of Sir Isaac Newton that white light is made up of light from all the colours of the spectrum: red, orange, yellow, green, and blue. Newton added indigo and violet to make the number up to seven for occult reasons. Light at the blue end of the spectrum is scattered much more than light at the red end of the spectrum. The red light mostly carries on in a straight line, the light that is scattered towards you, which you perceive as actually coming from the sky, is blue. And that's why the sky looks blue.

At the beginning or the end of the day, when the light that reaches you from a sun that is low in the sky has travelled a long distance through the atmosphere, most of the blue has been scattered from the light before it reaches you, which is why the dawn and dusk skies look red in the direction of the rising or setting sun, although the sky directly above you may still look blue. The colour of the light from the sun hasn't changed, it is how it is affected by interaction with the earth's atmosphere that makes it look different colours.

This page updated April 2019. W3CMVS.