What term describes a disk of optically transparent material?

• A. PMT
• B. Light pipe
• C. Crystal
• D. Collimator

Answer: (B)

A disk of optically transparent material functions as a light pipe. Its job is to carry light from one point to another by internal reflection within the clear material, acting as a conduit that channels light to a detector with minimal loss. The other terms refer to different roles: a photomultiplier tube detects and amplifies light; a crystal (as a scintillator) emits light when struck by radiation but is not simply a light-guiding disk; a collimator shapes the path of incoming photons rather than guiding light within a transparent medium. So the description fits a light pipe, the light-guiding disk.