What does the index of refraction measure?

The index of refraction is a fundamental concept in optics that quantifies how light propagates through different media. Specifically, it measures the ratio of the speed of light in a vacuum to the speed of light in a given medium. When light travels from one medium to another, its speed changes, and this change is what the index of refraction reflects.

A medium with a higher index of refraction has a lower speed of light compared to vacuum, meaning light travels more slowly through that medium. This characteristic is crucial for understanding the bending of light, or refraction, that occurs when light transitions between media with different indices of refraction. Hence, the correct understanding of the index of refraction not only aids in predicting how light will behave in various environments but also lays the groundwork for applications such as designing lenses and optical devices.