Refractometer

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.

An Abbé refractometer places a small sample between an illuminating prism and a measuring prism, then locates the boundary between bright and dark parts of an angular field. That limiting ray determines the sample's refractive index through Snell's law and the known measuring-prism geometry.

The prism surfaces must be clean and the sample must make good optical contact: a trapped air layer introduces a different interface. Because refractive index varies with wavelength and temperature, a useful reading specifies both; an instrument using white illumination needs compensation for the colored boundary caused by dispersion. A reference liquid checks the reading before an unknown sample is measured.