White light is a mixture of electromagnetic waves with different wavelengths. A monochromator is used to separate a single wavelength from an incoming beam of light. After passing the entrance slit of the monochromator the light is expanded into its wavelengths. This can be done by a dispersive element like a prism, or, like for KNAUER monochromators, by an optical grating, which diffracts the incoming light. With another slit at the outlet of the polychromator only a small wavelength range of the expanded light beam leaves the polychromator and enters the flow cell. As the outlet slit is fixed in its position it is necessary to rotate the grating in the monochromator to choose the desired wavelength for the HPLC analysis.