76.

From the foregoing it is seen that the frequency of all rays which strike the moving reflector are increased on reflection, when the reflector moves toward the radiation, and decreased, when the reflector moves in the direction of the incident rays ($v<0$). However, the total radiation of a definite frequency $\nu$ striking the moving reflector is by no means reflected as monochromatic radiation but the change in color on reflection depends also essentially on the angle of incidence $\theta$. Hence we may not speak of a certain spectral "displacement" of color except in the case of a single pencil of rays of definite direction, whereas in the case of the entire monochromatic radiation we must refer to a spectral "dispersion." The change in color is the largest for normal incidence and vanishes entirely for grazing incidence.