The index of refraction of a material is defined by the speed of light in vacuum c divided by the speed of light through the material v. Thus an electron in water can travel faster than light in water. The speed of light is maximum in.
C dt.
Next measure the speed of light in the medium. Refraction is the bending of the path of a light wave as it passes across the boundary separating two media. Speed of light in air or vacuum Speed of light in the medium. The index of refraction of a material is defined by the speed of light in vacuum c divided by the speed of light through the material v.