A diffraction grating is an optical element that divides light composed of lots of different wavelengths into light components by wavelength. It is an optical grating with a periodic structure that diffracts light into several beams traveling in different directions. When white light enters the grating, the light components are diffracted at angles that are determined by the respective wavelengths, and picking out diffracted (reinforced) light makes it possible to select the required light component.

Diffraction gratings are commonly used in monochromators and spectrometers, but other applications are also possible such as optical encoders for high-precision motion control and wavefront measurement. They are critical for a wide variety of applications including spectrometers, other analytical instruments, telecommunications, and laser systems.

There are two broadest categories of diffraction gratings: reflection and transmission gratings. Reflection gratings are essentially mirrors with microscopic grooves, and all diffracted orders reflect off of the grating at different angles. Transmission gratings are like lenses with microscopic grooves, and all diffracted orders transmit through the grating but are offset by angles following a specific equation.

Diffraction gratings are immensely useful tools for the separation of the spectral lines associated with atomic transitions. They are also used for measuring atomic spectra in both laboratory instruments and telescopes.