The measure of how much a light ray bends when it passes from one medium into another is called the index of refraction.

Here's a breakdown:

* Refraction: This is the phenomenon of light bending as it passes from one medium to another. This bending occurs because light travels at different speeds in different materials.

* Index of refraction (n): It's a dimensionless number that represents the ratio of the speed of light in a vacuum (c) to the speed of light in the medium (v).

* n = c/v

* Higher index of refraction: A higher index of refraction means light travels slower in that medium, and thus bends more significantly.

Example: Light bends more when going from air (n≈1.00) to water (n≈1.33) than when going from air to glass (n≈1.5).

Let me know if you'd like more details on how the index of refraction affects the bending of light!