The measure of how much a ray of light bends when it enters a material is called the index of refraction.

Here's why:

* Refraction: This is the bending of light as it passes from one medium to another (like from air to water).

* Index of Refraction: This is a number that represents how much a material slows down light compared to the speed of light in a vacuum. A higher index of refraction means the light bends more.

Here's a simplified explanation:

Imagine light as a car traveling on a road. When the car goes from a smooth highway (air) to a bumpy dirt road (water), it slows down and changes direction. The bumpier the road, the more the car's path changes.

Similarly, the higher the index of refraction, the "bumpier" the material is for light, and the more the light bends.