A diffraction grating is an optical element that divides(disperses) light composed of lots of different wavelengths(e.g., white light) into light components by wavelength. The simplest type of grating is one with a large number of evenly spaced parallel slits. When white light enters the grating, the light components are diffracted at angles that are determined by the respective wavelengths(diffraction). Picking out diffracted (reinforced) light makes it possible to select the required light component. In short, for parallel beams that enter neighboring slits, light is reinforced when the optical path difference, BD - CA, is a multiple of the wavelength. The light from all the slits is reinforced in the same way to produce "diffracted light."