Which term defines complete reflection of a light ray within a material?

• A. Complete reflection of a light ray within a material.
• B. The path length of light through the gem.
• C. The refractive index of the material.
• D. The polarization of light.

Answer: (A)

Total internal reflection is the phenomenon where a light ray traveling from a denser to a rarer medium is reflected back into the original medium when the angle of incidence exceeds the critical angle. The description “complete reflection of a light ray within a material” matches this phenomenon exactly, so it’s the correct term. When the incidence angle is below the critical angle, some light refracts into the second medium instead of reflecting. Polarization and refractive index describe different properties or effects (orientation of the electric field and how much light bends, respectively) and do not define the occurrence of total reflection. Understanding this helps explain why gemstones can appear so bright, as light is trapped and redirected within the cut facets.