It was Sir Isaac Newton who first investigated the colour content of white light. He passed a narrow beam of sunlight through a triangular prism and observed the spread of colours we associate with a rainbow. By selecting any one of the resulting colours and showing that, when it was passed through a prism, no further change occurred he showed that white light was a mixture of colours. We call the rainbow spread of colours a 'spectrum'. A spectrometer is a small telescope-like instrument containing a prism (or equivalent diffraction grating) that splits light into its constituent parts. Absence of a particular colour appears as a black line in the spectrum. Increase in the intensity of a particular colour appears as a heightening of the corresponding colour band.

The picture top left was taken through a spectrometer and shows the spectrum produced from the light of a 40-watts spotlight. Above the spectrum, the corresponding wavelength in nanometres can just be seen as feint orange numbers.

The picture bottom right is the corresponding spectrum for an energy saving lamp and shows that some colours are clearly 'missing', and that the intensity of violet has been increased.

The difference between the two spectra emphasises the importance of using a good light source when trying to reproduce faithfully the colours in a fossil or mineral specimen.

Visible light is a miniscule part of a large, continuous spectrum of electromagnetic radiation. Beyond the violet end is ultraviolet radiation (Ultra=above), a constituent of sunlight, X-rays and gamma radiation. Beyond the red end of the visible spectrum is infrared radiation (Infra=below) sensed as heat, and radio waves.