Explaining the Colours of the Spectrum to your Child
By David Lillis
28 February 2017
When your child turns nine or ten years of age, perhaps you may wish to interest him or her in science. Recently, my son turned eleven and, like all kids of his age, he has been exposed to some elementary science and mathematics at school. Like many people trained in the sciences, I have tried to guide him a little in mathematics and science, and have sought to extend him beyond the regular school curriculum in algebra and geometry. I have even taught him some simple computation and coding in the statistical and graphics programming language R (which originated here in New Zealand) and introduced him to Excel, teaching him about Excel functions, continuous and categorical variables, filtering data, sorting data and pivot tables, on the basis that skills in Excel will prove invaluable later on. In describing science or mathematics to my son, I have no hesitation in using Google, and in general I believe that there’s no major harm in using Wikipedia for kids!
The other day at Westfield Plaza, Lower Hutt, I bought a 64 mm triangular prism made by the US company The Light Crystal (http://www.sciencealive.co.nz/scienceshop/the-light-crystal). I wanted to tell my son about some basic science, in this case the splitting of light into the colours of the rainbow, allowing him to use the prism to split light, and adding human interest through the story of Isaac Newton and his scientific work at the time of the plague.
This epidemic occurred in 1665 and 1666, and was the last major outbreak of the bubonic plague to occur in England (though smaller scale outbreaks did occur in later times). Wikipedia tells us that this particular epidemic killed perhaps 100,000 people, roughly a quarter of London’s population, in a space of 18 months. The plague was spread by the Yersinia pestis bacterium, which is transmitted through the bite of an infected rat flea. Actually, Newton was forced to leave Cambridge for two years because of the plague, and so his work on the splitting of light was put on hold for that period. I thought that this story, even with its regrettable outcome for so many people, could nevertheless provide historic context for a young child encountering a fundamental principle of science for the first time. And so – after talking about the plague, we got to the topic of the colours of the spectrum!
We browsed the Internet and sourced the above two pictures using Google Images – Newton splitting light and a nice diagram of light refracting through a prism.
The following two photographs were kindly sent to me by Brian Jones, member of Council of the Royal Society of New Zealand Wellington Branch.
Here we see Woolsthorpe Manor in Lincolnshire, where Newton stayed when Cambridge was closed by the plague and where he did most of his work on light. He had a little bedroom and study on the first floor, with a big fireplace for warmth. The living space was separated from the hay loft by a simple clay plaster wall. There was no bathroom or toilet as we know it. The toilet was outside, and the only washing facility would have been a large bowl.
The apple tree in the photograph above is supposed to be THE tree – whose falling fruit provided Newton with the clue to gravity.
The point is that Newton didn’t have a fancy laboratory. His family was very poor, even by the standards of his day. However, he used observation and had a questioning mind – attributes that we all have if we wish to use them!
Back to our discussion of Newton’s work! We also used Google to source some of the text of Newton’s original work on dispersive refraction (Philosophical Transactions of the Royal Society, 1671, page 682). A very good web-site to source Newton’s original writings on optics and which provides clear accounts of Newton’s work is this one from the University of New South Wales (UNSW):
http://www.animations.physics.unsw.edu.au/jw/light/Newtons-prisms.htm
Let’s quote from Newton:
I procured a triangular glass prism, to try therewith the celebrated phaenomena of colours. And for that purpose having darkened my chamber, and made a small hole in the window shuts, to let in a convenient quantity of the sun’s light, I place my prism at his entrance, that it might be thereby refracted to the opposite wall. It was at first a very pleasing diversion to view the vivid and intense colours produced thereby.
A little later in the same text we learn that certain laws of refraction were already around at the time of Newton, but the challenge was to explain the laws of refraction to my own child. So, we took the prism outside on a very sunny morning in our back garden in Eastbourne. There he held the prism to the sun and created his first spectrum.
Here he is, casting a spectrum very successfully onto a painted wooden post on the morning of Saturday 25 February 2017. In the above photograph, he is holding the prism in such a way as to cast the component colours in reverse order, and thus blue and violet appear above orange and red. However, that morning he could see that blue and violet light was refracted more than red and orange.
Others before Isaac Newton had known that a prism separates sunlight, that oil on water gives rise to concentric rings of colour, and that bubbles can do something similar. Today, we have clear descriptions on how multiple reflections, refractions and transmission of sunlight through raindrops give rise to rainbows. My first clear description of the formation of rainbows was kindly provided by Professor John Lekner (of Victoria University) in an advanced physics class many years ago, and I think that I remember Professor Lekner telling us that, when we see a rainbow, in fact each of us has his or her personal rainbow because each of us observes from a slightly different location. I always emphasise this point to my son or to friends when we encounter a rainbow!
However, Newton was the first to explain clearly that it is refraction that causes sunlight to separate into its various colours (frequencies or wavelengths) and that each frequency or wavelength is refracted at its own unique angle.
Newton also showed that a second prism could recombine the colours to produce white light, and he showed that each colour is unchanged when passed through a second prism. So, while white light can be broken into component colours, none of those colours can be decomposed further. That’s interesting!
After Newton it became clear that white light is composed of light of different colours (frequencies and wavelengths), and that each colour is refracted differently. The different colours are the manifestation of light of particular frequencies or wavelengths, and the colours are refracted at different angles after passing through the prism. We refer to the separation of colours as dispersion.
In the UNSW web-site we learn that the medieval rainbow had only five colours: red, yellow, green, blue and violet, but that Newton added orange and indigo:
“ — so that the colours would be “divided after the manner of a Musical Chord”
(I. Newton in Opticks 4th edn, 127 (William Innys, 1730)”.
Though not addressed in Newton’s work (at least not addressed by him in any text or web-page I have seen), I nevertheless explained to my son some physics that is commonly introduced at senior secondary school – the notion of refractive index. Any sixth form or NCEA Level 2 Physics text will tell you that the refractive index (n) of a substance, like glass, water or air, depends on the speed at which light travels in that substance, relative to its speed in air. Specifically, it gives the ratio of the speed of light in a vacuum (c) to the speed of light in the substance (v). So, we have a simple formula for the refractive index that even a young child can understand:
n = c / v
Now we can explain that the refraction (bending) of a particular frequency or wavelength of light, as it passes from one substance to another (e.g. from air to the glass prism), depends on its refractive index, which itself depends on the extent to which the light slows down in the second substance.
The two of us enjoyed the following videos:
https://www.youtube.com/watch?v=uucYGK_Ymp0
and
https://www.youtube.com/watch?v=Aggi0g67uXM
and
https://www.youtube.com/watch?v=q73VNpFA-0Q
That’s as far as I went and perhaps that’s about as far as we can go with a young child who is not yet of secondary school age and who is encountering the splitting of light for the first time. Actually, when I purchased the prism, I also purchased a gyroscope with the intention of explaining angular momentum – but that’s another story!
Please feel free to give feedback on introducing science to kids.
David Lillis
25 February 2017
The author wishes to thank Chrissy Boulton, Lucy Forde and Brian Jones for contributions to this blog.
