• Red: 750 – 620 nm
• Orange: 620 – 590 nm
• Yellow: 590 – 570 nm
• Green: 570  -495 nm
• Blue: 495 – 450 nm
• Violet: 450  -380 nm

Here I’ve used the figures given by Wikipedia (sourced from catchily titled the CRC Handbook of Fundamental Spectroscopic Correlation Charts). The same ranges are shown on the image below.

You might notice how narrow the Yellow and Orange bands appear and, if you’re a keen colour cadet, that Indigo isn’t on the list at all. The rhyme I learned at school was Richard Of York Gave Battle In Vain – seven colours in the order of they appear in the spectrum. In fact Indigo used to lie between 450 and 440 nm. That is a really tiny waveband. Why? Because most people cannot tell you the difference between indigo and either blue or violet. Essentially the colour is more akin to one of those Dulux paint charts from the DIY store that it is to any of the ‘real’ basic colours. You may as well include biscuit, lavendar and lime in the spectrum.

Indigo was originally included in the spectrum by Isaac Newton, the man who formally discovered that light is split into different colours. Using a prism he showed that white light was a mixture of what he saw as seven basic hues, including indigo. He chose seven because he was attempting to make the colours of the Universe fit in with what he was as a numerological pattern appearing elsewhere.

Isaac Newton was not what you would recognise today as a scientist. He believed in alchemy (the idea that you can convert one element into another, e.g. lead into gold) and was extremely mystical. He was quite eccentric and didn’t even publish his most famous works until long after he had finished working on them.

At the time when he split light into a spectrum there were seven known planets in the Solar System. there were seven musical notes (do ra me fa so la and te) and of course there are seven days in a week –  a Biblical determination. So seeing his spectrum as being made of seven colours was quite a natural idea and he made it fit. The image at the top of this post shows his original colour circle – musical notes are shown around the edge, matching certain colours.

It got me wondering how many colours I would have divided the spectrum into if I had been the first to properly measure it. Ten colours, to match my decimal upbringing – maybe 5? Perhaps I would have added octamarine? It would be interesting to get children to divide up the spectrum themselves and then compare it with Newton’s result. How many colours can you see?

The story of Newton ‘inventing’ indigo is one I heard a while back but dismissed. It came back to my mind last night after it was mentioned on BBC Radio 4′s The Unbelievable Truth. They also mentioned another Newton tale which I shall finish up this post with. Newton spent a short part of his life (1689-1690 and 1701-1702) as a Member of Parliament for Cambridge University. During his two-year tenure he is only recorded to have made one speech: he asked for the window to be closed because he was cold. I think that tells you a lot about him.

Spectrometers are used, like prisms, to spread light out into the component colours. This enables us to understand the compositions of everything from stars to streetlights. Here I show you how to make your own spectrometer and give you a few examples of what you can see with it.

What You Need:

• A cardboard tube (toilet roll or kitchen roll tubes are just perfect, in the pictures here, I have used more black card to make a tube myself.)
• 2 square pieces of black card (approx 8cm x 8cm)
• Black tape or masking tape (something that blocks out light)
• Razor blades (nothing fancy just cheap blades that are not attached to anything)
• A stanley knife
• An old CD

Make a Diffraction Grating:

Cut a small square hole (approx 1cm across) in the middle of one of your 8cm x 8cm black cards. Break the CD into pieces, just snap it. You’ll need a section of the broken CD that can nicely cover the 1cm hole in your card.

Using a bit of sticky tape, peel away any cover remaining on the piece of CD, so that it is transparent. Use tape to stick it over the hole, creating a sort of window. This will be our diffraction grating.

Make a Very Fine Slit:

Using a stanley knife, cut a slit in the middle of the other piece of 8cm x 8cm black card. This slit should be about 2cm long and just a few millimetres wide. Tape the the two razor blades either side of the slit. They should make an even narrower slit, just 1mm or less if possible!

The aim is create a very fine, narrow slit though which light can travel. Make sure the blades are securely attached with tape for safety.

Make the Spectrometer:

This is the easy bit! You now attach the two square cards to either end of the tube using the dark tape. You have to attach it in such a way that no light is let into the tube accidentally (hence the dark tape). When you look through the diffraction grating, you only want to see light coming from the slit.

Testing Out Your Spectrometer:

The best way to see how this works is to use daylight. Just point the spectrometer toward a window during the day or up at a cloud if you’re outside. You should not ever look directly at the Sun. You should see a nice, smooth spectrum (rainbow) somewhere in your field of view in the tube. Here is a photo of a cloud taken through my own spectrometer. The bright white light is the slit and spectrum is just off to one side.

What’s Happening?

When light enters the tube though the slit it spreads out – all waves do this when passing through small slits. The CD then makes the separate colours visisble to your eye. You see a nice, even spectrum from daylight sources because daylight is made up of all the colours of visible light from the Sun. Once you can see this pattern, you can start trying to find the spectra of other things.

In our physics lab we have lamps of different chemical make-ups. These let us see pure light from different sources. Here are a few I took today, all photos taken by my own camera through my own, homemade spectrometer.

Here is the spectrum for Zinc, which you can see contains some red and blue but very little green.

Cadmium is very distinctive, with short sections of each of the three primary colours and very little between them. It is less spread out than Zinc. There is a big gap between the green and red sections.

Krypton is seen to be fainter than the others here, but the spectrum is still visible. The blue section has become much more violet or indigo here and the green is greener than it was in Cadmium.

The Astronomy Connection:

This is how astronomers know what stars are made of. They use advanced spectrometers to measure the spectrum of stars and pull out the ‘fingerprint’ patterns of colour that you see above. Each element has a unique set of spectral lines (colours) and these can identify the presence of different chemicals in stars, nebulae and just about everything else.

This is the whole spectrum of the Sun. It is so detailed that it had to spread onto multiple lines to see it properly! You’ll see that in fact it is not perfectly evenly spread out as I suggested earlier. This was taken with a very advanced spectrometer that has a greatly increased sensitivity compared to one made here, but its based on the same principles.

Things to Look At With Your Spectrometer:

• Sodium streelights
• Compare daylight to a lightbulb.
• Different light bulbs look different (that’s why energy saving bulbs light up the room in a different way).
• Neon signs.
• TV  and computer screens.
• LEDs from computers or remote controls (these give very pure spectra, often only one colour).

Have fun with your spectrometer and why not try and take a photo through it? It worked fairly well for me. I’d love to see any photos you take with it, or of it. Let me know how you get on. Thanks to the Science Made Simple team for this great idea!