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The refractive index is an important property of the components of any [[optical instrument]]. It determines the focusing power of lenses, the dispersive power of prisms, the reflectivity of [[anti-reflective coating|lens coatings]],<ref>{{Cite book |last=Willey |first=Ronald R. |url=https://www.spiedigitallibrary.org/ebooks/FG/Field-Guide-to-Optical-Thin-Films/eISBN-9780819478221/10.1117/3.668269 |title=Field Guide to Optical Thin Films |date=2006-01-27 |publisher=SPIE |isbn=978-0-8194-7822-1 |doi=10.1117/3.668269}}</ref> and the light-guiding nature of [[optical fiber]].<ref>{{Cite journal |lastlast1=Takeo |firstfirst1=Takashi |last2=Hattori |first2=Hajime |date=1982-10-01 |title=Optical Fiber Sensor for Measuring Refractive Index |url=https://iopscience.iop.org/article/10.1143/JJAP.21.1509 |journal=Japanese Journal of Applied Physics |volume=21 |issue=10R |pages=1509 |doi=10.1143/JJAP.21.1509 |bibcode=1982JaJAP..21.1509T |issn=0021-4922|url-access=subscription }}</ref> Since the refractive index is a fundamental physical property of a substance, it is often used to identify a particular substance, confirm its purity, or measure its concentration. The refractive index is used to measure solids, liquids, and gases. It can be used, for example, to measure the concentration of a solute in an [[aqueous solution]].<ref>{{Cite journal |lastlast1=Warren |firstfirst1=John R. |last2=Gordon |first2=Julius A. |date=January 1966 |title=On the Refractive Indices of Aqueous Solutions of Urea |url=https://pubs.acs.org/doi/abs/10.1021/j100873a507 |journal=The Journal of Physical Chemistry |language=en |volume=70 |issue=1 |pages=297–300 |doi=10.1021/j100873a507 |issn=0022-3654|url-access=subscription }}</ref> It can also be used as a useful tool to differentiate between different types of gemstone, due to the unique [[Chatoyancy|chatoyance]] each individual stone displays. A [[refractometer]] is the instrument used to measure the refractive index. For a solution of sugar, the refractive index can be used to determine the sugar content (see [[Brix]]).
