A teaching device is described which can be used in lectures on the interference of white light. The wave forms of four selected components of white light, in the violet, green, yellow, and red regions, are shown one above the other on a cardboard chart, inked in their respective colors. A sheet of clear plastic with an identical set of waves drawn on it is placed over the chart so that the two sets are in register. The combination represents a beam of white light as it arrives at the retina. A retardation of part of the beam is illustrated by displacing the plastic sheet laterally. Since the sine waves differ among themselves in wavelength, the various phase changes which result among the four components are shown quite clearly.The principle has been embodied in slide rule form with the wave trains appearing at a large plastic window. Five smaller windows indicate, respectively, path difference in millimicrons, dominant wavelength and purity of the resulting light when C.I.E. IlluminantCis used, and relative energies and resultant hue for the four components shown.