What is the index of refraction?

The index of refraction is fundamentally defined as a dimensionless number that indicates how fast light travels in a given medium compared to its speed in a vacuum. Specifically, the index of refraction (n) is calculated using the formula n = c/v, where c is the speed of light in a vacuum and v is the speed of light in the medium.

This means that if light travels slower in the medium, the index of refraction will be greater than 1. For example, glass has an index of refraction around 1.5, indicating that light travels 1.5 times slower in glass than in a vacuum. This property is critical in understanding how light bends or refracts when it enters different materials, which is an essential concept in optics.

Understanding this definition helps clarify why other options do not accurately characterize the index of refraction. While the speed of light in a vacuum is an important concept, it is not the definition of the index itself. Additionally, the ability of a material to reflect light is related to its reflectivity, and light absorption pertains to a material's absorption coefficient, neither of which describe the index of refraction directly.