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Murray Edwards College
University of Cambridge

School Winner: Why is the sky not violet?

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    05 Oct

    Science fact

    Elephants rarely get cancer: less than 5% of captive elephants die of cancer, compared to 20% of humans. Elephant genomes have at least 20 copies of the tumour suppressor, p53, which may explain their low cancer rates relative to humans, who have only one copy. 

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    As a child I asked an infinite number of questions: Why is it always raining? Why can’t I eat chocolate for breakfast? Why is the sky blue? Yet all these questions were answered with ‘‘it just is’’. However, through studying physics and other sciences I learnt that nothing ‘‘just is’’.

    So…. Why is the sky blue?

    Firstly, the sun emits energy as beams of light which are electromagnetic waves. When the beam of light approaches the earth the most harmful components of the beam, which have the shortest wavelengths and highest frequency (gamma, x-ray and ultraviolet waves), are prevented from passing through the stratosphere by the ozone layer. The ozone layer still allows radio and visible light waves, which have a larger wavelength, to pass through.

    The visible light that passes through the atmosphere is made up of all the colours of the rainbow. So why doesn’t the sky appear multicoloured? This is due to the scattering of the visible light waves as they collide with nitrogen and oxygen molecules in our atmosphere. The degree to which the visible light wave is scattered is dependent on the wavelength of its component parts. Similar to the waves you see on the beach some are larger while some are smaller. The colours red, orange, yellow and green have larger wavelengths while blue and violet have a smaller wavelength and a higher frequency. The smaller the wavelength of the light the more the light is scattered by the particles in the atmosphere. Thus, the light with the higher wavelengths pass through the atmosphere with little or no scattering, while blue and violet waves are more scattered.

    Finally, the blue and violet light waves which are scattered across the sky enter our eyes making the sky appear blue.

    However, this begs the question if violet light has a shorter wavelength and is scattered more, then why does the sky not appear violet?

    This is because the sun emits a higher concentration of blue light waves in comparison violet. Furthermore, as our eyes are more sensitive to blue rather than violet this means to us the sky appears blue.

    Science is a subject that is constantly evolving and nothing appears just as it seems and this is why it interests me, for example in class I have just learnt light is not just a wave it is also a particle which intrigues, perplexes and pushes me to keep asking questions.

    Darcey Smyth
    Victoria College

    Hi, my name is Darcey and I’m currently in year 13 and studying Physics, Biology, Chemistry and Maths in Victoria College Belfast in Northern Ireland. In two years’ time I hope to be studying Medicine or a career path related to Science at university. Also I believe that it is paramount for young women to study science as women are just taking a place in this vast, relatively male dominated field. Finally, I find studying science intriguing as every lesson you begin to understand a bit more about the world around us yet there are always more questions to be asked. In the words of Michelle Obama, ‘‘Focus more on learning than on succeeding. Instead of pretending that you understand when you don’t, just raise your hand and ask a question.’’