Answer: This is an optical refractometer.

An optical refractometer is a device used to measure the refractive index of a substance, which helps determine its composition or concentration. It operates by passing light through a sample and measuring the angle of refraction as the light exits the material. This angle is then used to calculate the refractive index, which varies depending on the substance's density and structure. Optical refractometers are commonly used in industries such as food and beverage (e.g., measuring sugar content in juices), pharmaceuticals, and chemical manufacturing to ensure quality and consistency in products.