When we look at a CD we see colourful stripes in addition to our own image. The CD works like a spectrometer. The light beams deviate in the grooves like in the chromatic glasses.

Depending on the source of light, different colours can be seen. In sunlight or lamp light we see all the colours of the rainbow. But if you take a fluorescent energy-saving bulb or a mercury one the colours would be 'incomplete' and would depend on the gas inside the bulb.

A CD shines with all rainbow colours because it forms a diffraction grid - the lines along which the information is written are equidistant. These lines, if illuminated, become sources of light on their own. Depending on the light colour (i.e. the light wavelength) and the observation angle, the intensity of light coming from these lines either sums-up or subtracts. Inclining little the CD, the colours change.

If a certain colour appears, a so-called constructive interference happens - the intensities of light emitted by single lines add-up. The condition for a constructive interference is that the differences between light paths starting from single lines amounts to an integer multiple of the wavelength font λ

d sin α = nλ .

where α is the distance between lines and k is an integer number (the image order).

The intensity of colours observed with CD is surprising. This is because the colours are observed in the reflected light - we see colours without a constant "background", differently than in the case of chromatic glasses (see before). To see well the colours in a CD one has to stand with his back to the light source, better if the source is distant and small, hold the CD with your arm straight and find the reflection of the source (still non colourful) and then turn around the disk slowly until colours appear.

The spectra obtained by a CD show also a high resolution, comparable to that of "real" spectrometers.

Here we can see two orders of difraction.

The photo shows the spectrum of a blue lamp, based on the electrical discharge in cadmium vapours, obtained with a CD spectrometer and with a professional one.