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The principle of the “CD Spectrometer” is quite well known; If you shine a thin beam of light (created by passing it through a narrow slit) at a CD, the light is reflected as a spectrum. The effect is caused because the CD acts as a reflection grating. There are hundreds of tiny reflections from the CD surface, which interfere with one another, sometimes adding together, sometimes canceling each other out. For each wavelength (colour), the pattern is slightly shifted so that you can see all the constituent colours of the source light.

Dominic (age 10) and I spent a day trying out this principle and “doing science”!

First we made the cereal box model MK I. It worked quite well but smelled of cinnamon!

To improve the design we built MK II using a tube from some kitchen foil. It seemed better because you put your eye right up to it, eliminating all other distractions.

This is MK III - it works really well visually and we had a bit of luck taking pictures of our spectrums (spectra?).

The slit is cut in some black electrical tape stuck over a thin acetate sheet; thin slits give better focus, but wider ones give brighter images. We found about 0.3 mm worked quite well.

If you use a whisky bottle tin, make sure it is single malt!

This version has a special reflector holder. The section of CD is stuck to a piece of foam board which pivots on a cocktail stick. This means fine adjustments can be made to the angle. About 60 degrees seems about right though. The “eyepiece” is just a piece of plastic tube from a light fitting. If you put your eye up close to the tube, only the spectrum light gets seen!

Here is Dominic demonstrating MK III.

It is a bit tricky to take pictures at the moment; I had to hand hold the spectrometer and the camera separately (I haven’t figured out a way to mount it on the camera yet).

To take a picture, I found that focussing at or about infinity seemed to work best. A digital camera is good because exposure times are a bit tricky to predict.

We were pleased with this spectrum! The sun emits white light (plus a lot of other radiation) but some chemicals present in the Sun’s “atmosphere” (outer layers) absorb certain wavelengths. The spectrum therefore has narrow dark lines in it corresponding to the chemicals present. These lines are quite easy to see visually but hard to photograph (for me). They are called Fraunhofer lines.

We found out what each of our lines represented from another website.

Eventually we hope to be able to calibrate our own spectrometer using known wavelengths, then we can use it properly to find out what things are made of!

Hot, glowing filament bulbs give off a continuous spectrum, but if you look at the results from other types of light, you get some interesting results.

Below is the spectrum from a “low Energy” light bulb. In both of these examples, gas is being excited by electricity. The bright coloured lines are called emission lines. Just like the Fraunhofer lines, they are a bit like a “fingerprint” for the chemicals involved.

We did try burning some chemicals like salt (sodium chloride). In theory you should be able to see the emission lines for the chemicals being burnt, but we didn’t have much luck...although we nearly started a fire!

We really enjoyed our experiments and plan to try and improve them. Why not have a go yourself?