Here's why:

* Refractive index is a measure of how much light bends when it passes from one medium to another. A higher refractive index means a greater bending of light.

* Speed of light is inversely proportional to the refractive index. This means that the higher the refractive index, the slower the light travels.

Example:

* Light travels fastest in a vacuum (refractive index of 1), where it reaches its maximum speed of approximately 299,792,458 meters per second.

* Light travels slower in water (refractive index of 1.33) and even slower in diamond (refractive index of 2.42).

Therefore, the medium with the highest refractive index, like diamond, will cause light to travel the slowest.