Answer:
a. the ratio of the speed of light in a vacuum to the speed of light in the material it is passing though
Step-by-step explanation:
When a light ray enters from one medium to another, it bends. This is because the speed of light decreases as it enters a medium having more optical density. The speed of light is maximum in vacuum.
The refractive index determines the optical density of a medium. It is the ratio of the speed of light in the vacuum (c) to the speed of light in the material it is passing through (v).
![\mu=(c)/(v)](https://img.qammunity.org/2020/formulas/physics/middle-school/yckdsb8zau6pw6mnmgjscan8j6fb4utbhj.png)