Which term describes the ability of a medium to slow down light?

• A. Optical Density
• B. Refraction
• C. Refractive Index
• D. Reflection

Answer: (A)

Light travels slower in most media than in a vacuum, and the quantity that describes how much that speed is reduced is the refractive index. It’s defined as n = c/v, where c is the speed of light in vacuum and v is the speed in the medium. This single number explains both the slowing and the bending of light at boundaries, since Snell’s law uses refractive indices to relate directions of light in different media. Optical density is a historical, qualitative term that loosely relates to how much a medium slows light, but it’s not the precise, standardized descriptor. Refraction describes the bending of light, not the property that governs its speed; reflection is light bouncing off a surface and also doesn’t describe the slowing effect.