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Colour of Crystals

Many crystals are clear and many have brilliant colours. Many of these crystals are insulators. To have colour they must absorb light, i.e. they must have electronic or vibrational transitions with energies equivalent to visible wavelengths, i.e. between 1.7 eV and 3.5 eV. Cadmium sulphide is yellow-orange because it has an energy gap at 2.42 eV, corresponding to 512 nm so that blue-green is absorbed.

Insulators generally do not have such transitions and so are clear, i.e. light can travel through them because they do not absorb energy in the visible region. Perfect diamonds are clear.

Colour in crystals is therefore often caused by impurities. Pure alumina Al2O3 is clear, but with small amounts of Cr3+ it becomes a dark red ruby, or with a small amounts of Ti3+ a blue sapphire.

Glass can be coloured by including fine particles to cause scattering of light at selected wavelengths, e.g. classic ruby glass is formed by the fine precipitation of gold in the glass.

For more information, see: C Kittel, Introduction to Solid State Physics, John Wiley & Sons, New York (1966), pp 537-8.

Colour on the Computer Screen

Natural colour is formed by the selective absorption of sunlight, i.e. an object appears blue because green and red are absorbed by the object and only blue is reflected by the object to our eyes. Such colour is called 'subtractive' colour.

Screen colour is formed by the mixing of pure colours: red, green and blue (RGB), in varying amounts in tiny regions of the screen. Such colour is called 'additive' colour. Natural colour and screen colour are therefore produced in different ways. This difference is important in understanding the difficulties of reproducing 'true' colour on a computer screen.

It isn't simply a matter of placing coloured dots close together, this can be done with paints as Georges Seurat and others did so beautifully during 19th century. The colour of the dots is formed differently, on the screen a red dot is formed by emitting red, in paint a red dot is formed by absorbing blue and green.

For more information, see: M E Holzschlag, "Chapter 5: Achieving High-End Color", in HTML Complete, Sybex, San Francisco (1999), pp 191-219.

First published on the web: 15 December 1999.

Author: Richard Payling