= Refractometer
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A <refractometer> measures <refractive index> through the way a sample redirects or reflects <light>. Some instruments measure the deviation through an <optical prism>; others locate the <critical angle for total internal reflection> at an interface with a prism of known <refractive index>.
In the latter geometry, for prism index $n_p$ greater than sample index $n_s$, the critical-angle relation is $\sin\theta_c=n_s/n_p$. Locating the transition between transmitted and totally reflected directions therefore determines $n_s$. Reference liquids check the angular calibration, while controlled <temperature> and <wavelength> make comparisons reproducible. Inferring composition from <refractive index> additionally requires a sample-specific calibration.
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