Which term is defined as a measure of the change in speed and direction as light travels from the air to a certain material?

• A. A measure of how much light slows down within a material.
• B. The overall condition of the facet surfaces of a finished diamond.
• C. A measure of the change in speed and direction as light travels from the air to a certain material.
• D. The exactness of the gem's symmetry.

Answer: (C)

This question is about how light behaves when it moves from air into another material. The term that captures how light slows down and changes direction in that transition is the refractive index. It’s defined as the ratio of the speed of light in air (or vacuum) to its speed in the material, n = c/v, and it governs the bending of light at the interface via Snell’s law (n1 sin θ1 = n2 sin θ2). Since this definition directly describes both the speed change and the directional change, it best fits the statement. The other options describe related but separate ideas—speed reduction alone, or gem surface quality and symmetry—which don’t capture the full concept of refraction.