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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".
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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
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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),
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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").
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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.
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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.
