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),
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