Nothing but blue

Nothing but blue
  • The truth is the sky isn't really blue, whle light from the sun is actually made up of seven different colours

Why is the sky blue? The truth is the sky isn't really blue; if it was blue then it would be blue all the time. White light from the Sun is actually made up of seven different colours - the colours of the spectrum: red, orange, yellow, green, blue, indigo and violet.

These colours can be seen in rainbows and pools of oil on water; splitting white light through a prism can also produce them. Light is made up of a wave similar to ripples on a pond. The distance between one peak to the next is known as the wavelength. In visible light we are talking about fractions of a millimetre. The different colours are produced because colours are at slightly different wavelengths with blue being the shortest and red being the longest.

Our atmosphere is made up of various gases, mainly nitrogen and oxygen with a few others mixed in. It also contains water vapour and dust. As the sunlight passes through the atmosphere, the longer wavelengths at the red end are not affected as much as the shorter wavelengths at the blue end which are scattered around in every direction.

This fills up the sky with blue light, especially at 90 degrees away from the sun. When you look in the direction of the sun, you see more of the light that comes directly from it and isn't scattered, so the sky around the sun looks whiter.

But at sunrise and sunset the sky can turn to a very deep red or orange. This is because the light now has to pass through more of the atmosphere and so the red end of the spectrum is more prevalent. The same effect can watching the full moon rising close to the horizon. The moon shines by reflecting sunlight off it and so this light has to pass through the thicker part of the atmosphere.

The UK experienced a pink sky and diffused orange Sun last Monday morning. This has been put down to sand from the Sahara desert mixed with smoke from the Portuguese forest fires being blown high into the atmosphere and carried by hurricane Ophelia as it battered the Irish coast.