Abbé refractometer
= Abbé refractometer
{c}
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.