What is the ratio of the speed of a wave in air to that in a material known as?

The ratio of the speed of a wave in air to that in another material is referred to as the refractive index. This concept is essential in understanding how waves, particularly light waves, behave when they transition between different media.

The refractive index quantifies how much a wave's speed decreases as it enters a material denser than air. It is calculated by dividing the speed of light in a vacuum (or air, as an approximation) by the speed of light in the material. A refractive index greater than one indicates that light travels slower in the material compared to air.

In practical applications, the refractive index is crucial for understanding phenomena such as refraction, lens design, and optical fiber technology, where controlling the propagation of light is essential for various technologies.