I assume that speed of light in a certain material could be stated as c1 em. The refractive index of an object is the ratio of the speed of light in a vacuum to the speed of light in the object. The refractive index of water n 133.
Next measure the speed of light in the medium.
The refractive index of an object is the ratio of the speed of light in a vacuum to the speed of light in the object. The bend caused by this slowing can be calculated based on the refractive index of the object. For example the speed of light through water is 22500000 kms. But nothing ever travels faster than c.