How to… make a DIY spectrograph
Read into the secrets of astronomical light
✔ Cereal box
✔ Old CD disc ✔ Sticky tape ✔ Modelling knife As you may know, light can be broken up into the colours of the rainbow, known as a spectrum.
This can be done at home using the properties of an old CD to break up the light. The spectrum will show you light and dark lines, which can also tell you what the sunlight, or other light source, is made up from.
Astronomers use similar devices, although somewhat more sophisticated, in their studies of the stars. The CD acts as a diffraction grating, that is, the circular tracks on the CD are so close together that they split up the light and spread each different wavelength to a differing position; this is the spectrum.
This spectrum is spread perpendicular to the CD – this is why the slit and the viewing holes need to be at 90° to each other. Each colour bends at a particular angle. For you to see the spectrum, the light must diffract off the CD and reflect into your eye. Adjusting the tilt of the CD allows you to properly bounce the spectrum into your eye.
The quality of the spectrum you get depends on how well you make the spectrometer. The slit which lets the light into the box needs to be thin, but not too thin, otherwise the image will be too dark. Conversely, if the slit is too wide, the spectrum lines will appear to be blurred.
If done right it is easy to use and a great way of getting into spectroscopy. It's a great tool for children, too, and is perfectly safe to use. You can also take pictures of the various spectra that you can see. This makes it easier to compare different kinds of light.
It's easy to make, although care should be taken, as you would using any sharp tool, when cutting the slit. You can use the spectrometer to examine light from other sources too, such as tungsten lights, strip lights and even the Moon. If you have a telescope, you could try using it on a bright star by holding the device over the eyepiece. You'll be able to spot the bright lines in the spectrum corresponding to various chemical elements in the light source. See what you can discover.
"The quality of the spectrum you get depends on how well you make the spectrometer”