The property of a material that indicates how much it reduces the speed of light is called the refractive index.

Here's a breakdown:

* Refractive index is a dimensionless number that describes how much light bends when it passes from one medium to another.

* A higher refractive index means that light travels slower in that material.

* The refractive index is related to the speed of light in the material by the equation:

* n = c/v

* Where:

* n is the refractive index

* c is the speed of light in a vacuum

* v is the speed of light in the material

For example, the refractive index of air is approximately 1.0003, while the refractive index of water is approximately 1.33. This means that light travels slower in water than in air.

Let me know if you have any other questions!