Purple cannot be represented by a single wavelength of light. No single wavelength exists for the color brown just as none exists for purple. Purple can be created with a mixture of wavelengths in both the red and the violet. Brown requires a more complex mixture of wavelengths from at least three regions of the sequence. Here we consider the color of an object illuminated by white light.
Color is produced by the absorption of selected wavelengths of light by an object.
Objects can be thought of as absorbing all colors except the colors of their appearance which are reflected as illustrated in Fig. 5. A blue object illuminated by white light absorbs most of the wavelengths except those corresponding to blue light. These blue wavelengths are reflected by the object. Figure 5 . White light composed of all wavelengths of visible light incident on a pure blue object.