What is the term for the ratio of wave speed in one material to that in another material?

• A. The index of refraction
• B. Young's modulus
• C. The acoustic impedance
• D. Poisson's ratio

Answer: (A)

The term that describes how fast a wave travels in one material compared to another is the index of refraction. For light, the index of refraction n is defined as n = c/v, where c is the speed in vacuum and v is the speed in the material. This tells you how much the material slows the wave. When you compare two materials, the speeds are related to their indices: v1/v2 = n2/n1. So the ratio of wave speeds across media is directly connected to the indices of refraction of the two media.

Other terms like Young's modulus or Poisson's ratio describe mechanical properties of materials (stiffness and deformation), while acoustic impedance relates to how much resistance a material offers to wave transmission. These don’t directly express the simple speed ratio between two media, whereas the index of refraction does.