What is a handheld refractometer, and how does it work?

A handheld refractometer is a portable device used to measure the concentration of a liquid. It uses the refraction of light to measure how dense a liquid is. When light passes from air into a liquid, it bends. The amount it bends depends on how much stuff (like sugar or alcohol) is in the liquid. The refractometer measures this bending angle and gives you a reading that tells you how concentrated the liquid is.