Because perception of colour stems from the varying sensitivity of different types of cone cells in the retina to different parts of the spectrum, colours may be defined and quantified by the degree to which they stimulate these cells. These physical or physiological quantifications of colour, however, do not fully explain the psychophysical perception of colour appearance. The science of colour is sometimes called chromatics. It includes the perception of colour by the human eye and brain, the origin of colour in materials, colour theory in art, and the
physics of electromagnetic radiation in the visible range (that is, what we commonly refer to simply as light).