Schematic diagram of a grating monochromator:

Within grating monochromator, a fine beam of the light from the source falls on a concave mirror by an entrance slit. This is then reflected on the grating that disperses it. The dispersed radiation is then directed to an exit slit. The range of wavelengths isolated through the monochromator is determined through the extent of dispersion through the grating and the width of the exit slit. Rotation of the grating within a predetermined way could be used to acquire the desired wavelength from the exit slit.

Figure: Schematic diagram of a grating monochromator