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
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.