aPigment,  Dye  and Stained Glass

Pigment works by absorbing some light and reflecting other light.  The reflected light is what we see.   Most objects have this type of colour.  The selection of the colours is dependent on the substance.  The atoms or molecules of a substance can possess only certain values of energy.  Photons which have exactly the right energy to move electrons up from one energy level to another have a high probability of being absorbed.  If the electron falls back to the original level, a photon of the same energy is emitted.  So many substances absorb the same radiation they emit.  For example, sodium emits light with two very closely spaced frequencies in the yellow part of the spectrum.  The spectrum of sunlight has two dark lines at exactly the same frequencies.  The spectrum of sunlight in fact has thousands of dark lines.  They are not really dark – just less bright than the surrounding parts of the spectrum.

Atoms, of course behave differently in crystals or in compounds from the way they behave alone in a gas. 

Glass can be coloured by very small mounts of additive.  The colour may depend on the type of glass or on the chemical state of the additive.  In all cases, the additive absorbs a range of frequencies, leaving the remainder to determine the colour we see.  It is not what we see that is characteristic of the additive – it is what we don’t see.  So if the glass is illuminated by different coloured lights, the additive will absorb whatever it finds of its own colours, and what we see may depend strongly on what we shine on the glass.

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