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What are the causes of colour?

Dietrich ZawischaHome/Contact

Colours are seen when sources of light emit predominantly or solely only parts of the full visible spectrum. This happens e.g. in firework, neon signs (or other gas discharge tubes) as used for advertising, or the screen (cathode ray tube) of a computer or TV-set. This is dealt with in the sections "glow and fire" and "atomic spectra".

Colours are seen when light, coming from somewhere (mostly the surface of some object) and reaching the eye, differs in its spectral composition from that of the light source. This may happen when part of the light is absorbed while the rest reaches the eye (this is the most common cause of colour and is dealt with in the sections on dyes and pigments and minerals),
or when the light, which in general is a mixture of waves of different wavelengths, undergoes some process which separates the different parts according to their wavelengths, e.g. by reflecting them into different directions as in the rainbow (this is dealt with in the sections "scattering", "diffraction and interference", and "refraction and dispersion").
Colours are seen when the retina of the eye is irritated "inadequately" (e.g. due to rubbing the eyes or pressure from outside on the eyeball) or when it is recovering after being dazzled or just regenerating after strong visual impressions (afterimages). Irritation of those parts of the brain which are involved in vision may lead to colour illusions (e.g. the fortification illusions accompanying an attack of migraine), or flickering change of light and darkness (e.g. when looking at the well known Benham top). These physiologically caused illusions are mentioned only, but will not be treated in detail here.

Any sensation of colour can be attributed to one of the above categories. These can be divided further according to the underlying physical processes which yields dozens of possibilities which should be dealt with separately.

The following sections are devoted to the underlying physics. This is fairly simple in some cases – but the colours seen most frequently are not easily understood, if one is not satisfied with explanations like "the green colour is due to the chlorophyll contained in the leaves". Why is chlorophyll green? Because it absorbs light with short wavelengths (blue) and with long wavelengths (red), but cannot absorb in the middle range (green). And what is the reason for that? Is it possible to tell from the chemical formula of a substance whether it is a dye and if so, of which colour?
In contrast to the colour phenomena occurring due to diffraction, refraction or interference (which are the topics of the later sections), this cannot easily be found out and tested by simple computing, and is far beyond the reach of these pages. Even for qualitative explanations, the equipment of modern physics, namely quantum theory is needed. Thus the chapters on dyes and pigments cannot be an easy reading, nevertheless, I hope they will be of some use for the interested reader.


Seeing, measuring, and rendering colours