A refractometer works on the principle of refraction, the bending of light waves as they pass from one substance into another. A familiar example is that of looking into the water at a submerged object and reaching for it. The object is not where it appears to be. This is due to refraction, or the bending of light waves, as they pass from water into air or from air into water. A refractometer is an instrument that measures this effect.

One refractometer configuration includes a so-called illuminating prism. The illuminating prism allows light to be introduced to the sample. Its function is to scatter the light so that the light source illuminating the sample is coming from many different directions. (Light traveling perpendicular to the surface will not be refracted at all, but will continue on a straight path.) Another component is the so-called refracting prism.

Sandwiched between the illuminating prism and the refracting prism is the sample being measured. Once the light source is turned on, this whole configuration essentially does the same thing as the example of looking into the water. The difference is that the refractometer measures the degree of light bending and assigns a number to it. The number is known as the refractive index.

Factors that influence refraction include temperature and the physical properties of the medium through which the light is traveling. Some refractometers include a thermometer to account for temperature. Since an ordinary light source for a refractometer would cause a problem we might call the rainbow effect, a monochromatic light (that of a single wavelength) is used as a light source. Rainbows, although they may be beautiful in nature, interfere with the refractometer's accuracy. Some refractometers can compensate for this with a set of prisms designed for just that purpose. In these instruments, ordinary multiple wavelength light can be used as a light source.

Refractometers are used in a variety of applications in industry and medicine. One example is that of making maple syrup. The maple syrup producer will place a sample of the maple sap onto a refractometer. Periodically checking the so-called refractive index as the sap is becoming closer to being syrup allows the producer to get it just right. In medicine, the refractometer is used for diagnostic purposes to determine the refractive index of blood and urine.