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‘Colour’ for the physicists is thus in some way a property of light and the object only matters in so far that it interacts with light hitting it and selectively reflects or absorbs light of a certain wavelength. Either the additive or the subtractive colour system allow us to create all the colours found in the visible spectrum but real objects are seldom emitting light at a single wavelength, nor are they composites of colour filters. On the contrary, most sources of light emit a mix of light at different wavelength. The intensity (measured in radiance) of the light emitted from a source at a given wavelength is described by the emission spectrum of the source and some examples have been shown below.

Objects on the other hand generally do not emit any light but most objects selectively absorb incoming light. The analog to the emission spectrum for objects is thus the absorption spectrum which describes the percentage of incoming light of a given wavelength that is absorbed by the object. Together with the emission spectrum of a light source we can use the absorption spectrum of the object to predict the wave length distribution and corresponding intensity levels of light from the source after it hit the object. Click here for an example of such a process.

Despite all the applications and predictions of this theory of colour, we still haven’t seen any fundamental reason why light with a wavelength of 632nm appear read. For that explanation we need to follow the light into the human eye.