Because light travels at different velocities in different mediums, the light beam is bent at the interface between those two materials. This is called refraction. There is an angle of light propagation at which refraction no longer occurs, the light beam instead being reflected back into the original material. This is called the critical angle. The PR-111 measures this critical angle, thereby inferring the refractive index of the second material.
Critical angle is the result of light interfacing at two mediums.
Critical angle is dependent on these RI's. Inside the probe is the optical block. The optical block contains the key components of the PR-111. The key components are the LED, prism, thermistor, and silicon detectors. During a measurement, the LED shines through a sapphire prism.