Monday 11 December 2017

Why is the sky blue?

Dr Robert Howard: Our blue sky and red sunsets are caused by the sun's light being scattered by all the atoms and molecules in the atmosphere. .

To help visualise this, imagine a bizarre rugby match in the Aviva Stadium. Ireland is playing a mixed team from Scotland and Wales. The Scots are small-sized players and wearing blue shirts, while the larger sized Welsh players are dressed in red. Ireland is defendingY

ou are a spectator watching the match from the sideline while your friend is watching the match from behind the posts.

The blue and red players start their attack from their end of the pitch and are running towards your friend, but past you. The Irish players manage to tackle and scatter the smaller blue players to the side lines but the larger red players push past them and make it down the full length of the pitch to the posts. From where your friend is sitting all they see is a wave of red coming towards them, while you being on the side line only see a wave of blue players being scattered towards you.

This unusual rugby match makes a good analogy for the scattering of light by atoms in the atmosphere.

The sun's light is made up of all the colours (from blue to red). The blue light has a small wavelength (450 nanometres, or 450 billionths of a metre) while red light has a bigger wavelength (650nm).

As the sun's light passes through the atmosphere it is scattered by the small atoms. The further it has to travel through the atmosphere the more blue light gets scattered out leaving only red light remaining.

So if your friend is in China enjoying a beautiful red sunset the light he sees has travelled much further through the atmosphere (because of the Earth's curvature) and is mostly red because the shorter wavelength blue light has been lost.

Meanwhile if you are in Ireland at noon the scattered light is mostly blue because the atoms preferentially scatter shorter wavelengths.

Similarly, at noon in China, when it is facing the sun, the skies are blue, while in Ireland the sun is rising.

Back to the match, when you're looking down the full length of the pitch (sunset), you see a wave of red, when you're on the side line (noon) you mainly see scattered blue shirts.

Irish Independent Supplement

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