A traditional handheld refractometer is an analog instrument for measuring a liquid's refractive index. It works on the critical angle principle by which lenses and prisms project a shadow line onto a small glass reticle inside the instrument, which is then viewed by the user through a magnifying eyepiece.
However, bench and handheld refractometers are usually used for more pragmatic purposes, usually to measure the concentration of a dissolved substance. The simplest and most popular use of a bench or handheld refractometer is in measuring the concentration of sugar in water.

A traditional handheld refractometer is an analog instrument for measuring a liquid's refractive index. It works on the critical angle principle by which lenses and prisms project a shadow line onto a small glass reticle inside the instrument, which is then viewed by the user through a magnifying eyepiece.
However, bench and handheld refractometers are usually used for more pragmatic purposes, usually to measure the concentration of a dissolved substance. The simplest and most popular use of a bench or handheld refractometer is in measuring the concentration of sugar in water.