Freshnel’s law:

These detectors based on Freshnel's law of reflection have the advantage of responding to most solutes and have a wide range of linearity where one cell covers the entire refractive index range. The cell volume is 15-25 µ L. In addition, these are highly reliable and remain unaffected by flow rate. However, these are temperature sensitive and do not yield sensitive results. Reflection category refractometer measures the modification in percentage of reflected light at a glass-liquid interface as the refractive index of the liquid changes. There are two collimated beams from the projector light illuminate the reference and the sample cells made of Teflon gasket clamped between the cell prism and a stainless reflecting back plate. Since the light beam is transmitted by the interface, it passes through the flowing liquid film and impinges on the surface of the reflecting back plate. This diffuse reflected light appears as two spots of light that are imaged by lenses onto dual photo detectors. Since the ratio of reflected light to transmitted light is a function of the refractive index of the liquids, the illumination of the cell back plate is a direct measure of the refractive index in each chamber. With mobile phase flowing through both compartments, coarse zero is adjusted by rotating the entire projector assembly. Fine adjustment is completed with the optical plate, a glass plate that could be rotated ±30^o from normal. Two different prisms must be used to cover the useful range of refractive index.