The property of a material that indicates how much the speed of light changes as it passes through is called the refractive index.

Here's a breakdown:

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

* Higher refractive index means light slows down more as it enters the material, causing a greater bend in its path.

* Lower refractive index means light slows down less, resulting in less bending.

For example, glass has a higher refractive index than air, which is why light bends when it passes from air into glass, causing things to appear magnified.

Let me know if you have any other questions!