David Boettcher - freelance engineer

Eur Ing D B Boettcher BSc(Hons) CEng MIEE MIET

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Why Is The Sky Blue?

Over the years I have "encountered" various explanations about why the sky is blue, but it was only recently that I realised that none of them really explained it clearly but instead relied on vague allusions to "scattering", so I decided to find out more. This is my simple take on why the sky is blue; it helped me to understand why it is, I hope it might help you.

Blue sky
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.

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 us 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 that make up the air 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, blue, indigo and violet. However, it turns out that light at the blue end of the spectrum is scattered much more than light at the red end of the spectrum, so the majority of 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.

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This page updated November 2015. W3CMVS.