A diffraction grating is a piece of glass or metal with a series of closely-spaced lines etched onto its surface. Typically there are 12,000 - 25,000 or more lines per inch (5000 - 10,000 lines per centimeter).

Light passing through the gaps between the lines are diffracted to a precise degree, resulting in multiple images to both sides of the central image. Since the angle at which the images are displaced from the center is related to the wavelength (color) of light, a diffraction grating produces a spectrum (or multiple spectra) from a white light source.

Long wavelengths (red) are diffracted more than short wavelengths (blue), so the order of colors in the spectrum produced is reversed compared to the order of colors in a spectrum produced by a prism.