Diffraction Gratings
A diffraction grating is a periodic structure that disperses light into its component wavelengths through multiple-beam interference, central to spectroscopy.
Definition
A periodically structured optical element that diffracts incident light into discrete orders at wavelength-dependent angles set by the grating equation, used to disperse and analyse light by wavelength.
Scope
This topic covers the physics and use of periodic diffracting structures. It includes the grating equation relating diffraction angle to wavelength, grating order, and groove spacing; the distinction between transmission and reflection gratings and between amplitude and phase gratings; blazing to concentrate light into a chosen order; and the spectral resolving power and free spectral range that determine performance. It treats gratings as the dispersing elements of spectrometers and monochromators and as components in pulse compression and beam control.
Core questions
- How does the grating equation determine the diffraction angles for each wavelength?
- What sets the spectral resolving power of a grating?
- How does blazing direct light into a useful diffraction order?
- How do gratings disperse light differently from a prism?
Key concepts
- grating equation
- diffraction order
- groove spacing
- spectral resolving power
- free spectral range
- blazed grating
- transmission and reflection gratings
- dispersion
Key theories
- Grating equation
- Constructive interference from many equally spaced grooves occurs when the path difference between adjacent grooves is an integer number of wavelengths, giving diffraction angles that depend on wavelength, order, and groove spacing.
- Resolving power and blazing
- The resolving power of a grating equals the product of the diffraction order and the number of illuminated grooves, while shaping the groove profile, or blazing, concentrates diffracted energy into a chosen order for efficiency.
Clinical relevance
Gratings are the dispersing elements in the spectrometers used for clinical chemistry analysers, blood-gas and oximetry measurements, and spectral-domain optical coherence tomography, where they separate light into wavelengths for analysis.
History
Rittenhouse made an early grating from fine wires in 1786, and Fraunhofer produced ruled gratings in the early nineteenth century to study the solar spectrum, discovering the absorption lines named after him. Rowland's precision ruling engines in the 1880s produced large, accurate gratings that transformed spectroscopy.
Key figures
- Joseph von Fraunhofer
- Henry Augustus Rowland
- David Rittenhouse
Related topics
Seminal works
- hecht2017
- bornwolf1999
Frequently asked questions
- Why does a grating produce several spectra at once?
- The grating equation is satisfied for multiple integer orders, so light of a given wavelength is diffracted into several directions, each corresponding to a different order and producing a separate spectrum.
- How is a grating better than a prism for spectroscopy?
- A grating's dispersion is more nearly linear in wavelength and its resolving power can be made very high by using many grooves, whereas a prism relies on material dispersion and generally gives lower resolution.