Moisture content of honey is an important property which impacts the shelf life and quality of the honey. The refractive index of honey varies as a function of moisture content. A simple instrument known as a refractometer may be used to measure the refractive index of honey. It uses the principle of total internal reflection. Light from a point source will either be refracted and pass through the prism-honey interface or be totally internally reflected if its angle of incidence on the prism-interface surface is equal to or greater than the critical angle. Therefore there will be a division between a bright and a dark field determined by the actual refractive index of the honey.
The refractometer may be calibrated using known samples, i.e. samples with known moisture content. Generally, there will be a simple screw mecahnism to shift the bright/dark division line to correspond with the appropriate moisture content on the scale. The response is reasonably linear over restricted ranges e.g. 10-25% for honey.