A spectrometer is a device that studies the composition of light. Yes, light is not actually white – it consists of many different colours. You can create your own spectrometer to marvel at the wonderful world of hidden colours behind everyday household objects such as light bulbs, fluorescent lights, computer monitors and candle flames.
You just need a CD to split white light into its different colours! You will see beautiful colours when light reflects off the surface of a CD. But why? The illustration above shows what a strong magnification of the surface of a CD would look like. The music is encoded in short and long pits, which are placed in a long spiral groove on the surface of the CD.
What you’ll need:
A cereal box
1 Pair of scissors
1 triangle ruler
How to do it:
- Cut a slit on one side of the box: On the top of the box, measure in 1.5 inches and make a mark.
- Cut along the guideline, then unfold the flaps you just made. Cut off the flaps.
- Place the short edge of the triangle along the top edge of the box and draw a 2,5 cm line towards the centre of the box: Using those lines as guides cut two 7 cm slits on both sides of the cereal box, like this:
5. Flip the box over and do the same thing on the other side.
6. Slide the CD into the slits.
7. Now you have to cut a rectangle out on the opposite long side of the box. The rectangle should be exactly 2 mm. Too wide and the spectra gets blurry. Too narrow then not enough light gets in.
The experiment begins! Try different kinds of light: sunlight, incandescent light, fluorescent light, LED light, even a laptop light!
Your first attempt is to look at the spectrum of an ordinary incandescent light bulb. This light shows a number of sharp lines against the background of a continuous spectrum. These emission lines are produced by low-density mercury vapour in the tube. The mercury also produces ultraviolet light, which is turned into a continuous spectrum of visible light by a thin layer of phosphor on the inside of the tube.
Secondly, tryfluorescent light! This one shows a very different spectrum. The reason is that manufacturers can vary the colour of the light by using different combinations of phosphors. The lamp illustrated uses a phosphor that emits a continuous spectrum, but this type uses so-called tri-colour phosphors: a combination of three phosphors which each has its own set of emission lines. We perceive the resulting mix of colours as white.
Candle light has a continuous spectrum. In the first few seconds after a candle (or a match) is lit, there is also the yellow sodium line which disappears thereafter. If table salt is burnt, the yellow sodium line becomes prominent. The sodium line should be double lines, but this simple device cannot resolve them.
Some of the spectra that can be seen with the cereal-box spectrometer have a continuous background, with the colours varying smoothly from dark red to dark blue. Others consist of sharp lines on a continuous background, sharp lines without background, or even dark lines on a continuous background.
Why all this diversity? How are spectra formed? It is all due to the atoms! An individual atom can find itself in different energy states, depending on whether its electrons occupy their usual orbits or have been excited to higher orbits. An atom cannot have just any energy: the energy levels are sharply defined, dictated by the detailed atomic structure. When an electron jumps from one orbit to another, the change corresponds to a precise amount of energy which is emitted in the form of a single photon. The energy of the emitted photon determines its colour. Cool, isn’t it?
Discover the power of light trying different ones and send us your own pictures! Go for it!