Q. Why is the sky blue?

Did you know?

We live in the Troposphere. It contains more than ¾ of the atmosphere.

The Sun is approximately 93 million miles away from the Earth. Sunlight takes 8 minutes 19 seconds to reach us.

C lick here to return to Q&As about Our Planet or use the search box below to surf...

A. Why is the sky blue must be one of the most commonly asked questions by children and adults alike.

So why is the sky blue? When we see something, whatever that may be – a wardrobe or curtains for example – light enters our eyes and our brain ‘interprets’ this and creates a picture. Most light comes from the sun.

Sunlight might look white but it is actually made up of the colours of the spectrum (think of a rainbow). Light travels from the Sun to the Earth in waves. Some light travels in shorter or choppier waves whilst others travel in longer or smoother waves. To help picture what we mean by waves, try thinking of a piece of cord. If you shake the cord up and down you will create a wave. A slow shake will produce longer smoother waves with fewer peaks and troughs whereas a rapid shake may produce more shorter waves within the length of cord. The distance between the various peaks is the wavelength. Different coloured light has different wavelengths – red has long waves, blue has short waves.

When sunlight enters Earth’s atmosphere it gets disturbed by the particles in the air (mainly oxygen and nitrogen) which scatters the light (in simple terms, light bumps into the atoms). Blue light is scattered or distributed more than red light because the wavelength of blue is roughly the same size as oxygen atoms. Not all blue light gets scattered but red light passes more easily through the atmosphere to reach Earth. This scattering of blue light throughout the atmosphere is the reason the sky looks blue - the blue light is hitting our eyes from many different angles. For reference, this phenomenon is called Rayleigh scattering after the British scientist who discovered this.

When the sun starts to set (or moves further away from us) light has further to travel, so much so that very little is able to pass through the extra distance and reach our eyes (in particular the short waves which are scattered even further a field). Both the sun and sky therefore seem less bright. Only the longer wavelengths such as red and orange now reach our eyes down on Earth, creating a beautiful reddish sun up above.

Source(s): Sciencemadesimple.com, NASA, BBC.co.uk, Sky-watch.com