Editing Refractive index (section)

===Homogeneous media===
{{Main|Refractometry|Refractometer}}
[[File:Pulfrich refraktometer en.png|thumb|alt=Illustration of a refractometer measuring the refraction angle of light passing from a sample into a prism along the interface|The principle of many refractometers]]

The refractive index of liquids or solids can be measured with [[refractometer]]s. They typically measure some angle of refraction or the critical angle for total internal reflection. The first [[Abbe refractometer|laboratory refractometers]] sold commercially were developed by [[Ernst Abbe]] in the late 19th century.<ref>{{cite web |url = http://www.humboldt.edu/scimus/Essays/EvolAbbeRef/EvolAbbeRef.htm |title = The Evolution of the Abbe Refractometer |publisher = Humboldt State University, Richard A. Paselk |year = 1998 |access-date = 2011-09-03 |url-status = live |archive-url = https://web.archive.org/web/20110612000645/http://www.humboldt.edu/scimus/Essays/EvolAbbeRef/EvolAbbeRef.htm |archive-date = 2011-06-12 }}</ref>
The same principles are still used today. In this instrument, a thin layer of the liquid to be measured is placed between two prisms. Light is shone through the liquid at incidence angles all the way up to 90°, i.e., light rays [[parallel (geometry)|parallel]] to the surface. The second prism should have an index of refraction higher than that of the liquid, so that light only enters the prism at angles smaller than the critical angle for total reflection. This angle can then be measured either by looking through a [[telescope]],{{clarify|date=June 2017}} or with a digital [[photodetector]] placed in the focal plane of a lens. The refractive index {{mvar|n}} of the liquid can then be calculated from the maximum transmission angle {{mvar|θ}} as {{math|1=''n'' = ''n''{{sub|G}} sin ''θ''}}, where {{math|''n''{{sub|G}}}} is the refractive index of the prism.<ref>{{cite web | url = http://www.refractometer.pl/ | title = Refractometers and refractometry | publisher = Refractometer.pl | year = 2011 | access-date = 2011-09-03 | url-status = live | archive-url = https://web.archive.org/web/20111020123455/http://www.refractometer.pl/ | archive-date = 2011-10-20 }}</ref>

[[File:Refractometer.jpg|thumb|alt=A small cylindrical refractometer with a surface for the sample at one end and an eye piece to look into at the other end|A handheld refractometer used to measure the sugar content of fruits]]

This type of device is commonly used in [[chemistry|chemical]] laboratories for identification of [[chemical substance|substances]] and for [[quality control]]. [[Digital handheld refractometer|Handheld variants]] are used in [[agriculture]] by, e.g., [[wine maker]]s to determine [[Brix|sugar content]] in [[grape]] juice, and [[inline process refractometer]]s are used in, e.g., [[chemical industry|chemical]] and [[pharmaceutical industry]] for [[process control]].

In [[gemology]], a different type of refractometer is used to measure the index of refraction and birefringence of [[gemstones]]. The gem is placed on a high refractive index prism and illuminated from below. A high refractive index contact liquid is used to achieve optical contact between the gem and the prism. At small incidence angles most of the light will be transmitted into the gem, but at high angles total internal reflection will occur in the prism. The critical angle is normally measured by looking through a telescope.<ref>{{cite web |url = http://gemologyproject.com/wiki/index.php?title=Refractometer |title = Refractometer |publisher = The Gemology Project |access-date = 2011-09-03 |url-status = live |archive-url = https://web.archive.org/web/20110910082406/http://www.gemologyproject.com/wiki/index.php?title=Refractometer |archive-date = 2011-09-10 }}</ref>