Light is a kind of electromagnetic wave. A chart of the electromagnetic wave spectrum is shown in Figure 1. It can be seen that visible light occupies a very narrow band of the whole spectrum. Typically, their wavelengths are between 4 x 10-7 m(4,000 Å or 0.4 m m) to 7 x 10-7 m(7,000 Å or 0.7 m m). ( Å stands for 'angstrom' and is equal to 10-10 m.)

	Lights with shorter wavelengths and hence higher frequencies have more energy than those with longer wavelengths and hence lower frequencies.
	By the use of special sources or special filters, it is possible to limit the wavelength spread to a small band, typically of a width of 100 Å. Such lights can be described as 'monochromatic', meaning of a single colour.