Which term is defined as the light returned when it hits a surface or a boundary between two different materials?

• A. The light returned when it hits a surface or a boundary between two different materials.
• B. The ability of a medium to slow down light.
• C. The change in speed and possible change in direction of light.
• D. The exactness of a finished gem's shape.

Answer: (A)

Reflection is the light that returns toward you after it strikes a surface or the boundary between two materials. This bouncing back of light is what we perceive as brightness and sparkle, and it’s essential to understanding how gems look to the eye. The law of reflection—angle of incidence equals angle of reflection—helps explain why certain cuts direct more light back to the viewer, enhancing brilliance. The other phrases describe different optical effects: slowing down light in a medium relates to refractive index and refraction, which is about light bending as it enters a new material. The exactness of a finished gem’s shape is a matter of cut accuracy and geometry, not the boundary behavior that produces reflected light.