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.
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