Unlike mechanical waves, electromagnetic waves can travel in a vacuum or in any material medium. Consider the electromagnetic wave passing through a dielectric medium. In such a case, Maxwell's equations get modified. In Ampere's law, ε0 is replaced with ε, the permittivity of the dielectric. Also, the vacuum permeability is replaced by the permeability of the medium. Recall the relationships between the electric and magnetic fields derived using Faraday's and Ampere's law. Since the speed of the wave in a dielectric is not the same as in a vacuum, the constant "c" is replaced with a reduced velocity "v". By comparing these two relations, the speed of an electromagnetic wave in a dielectric can be obtained. Except for ferromagnetic materials, the relative permeability of most dielectrics is usually close to unity, and the dielectric constant is always greater than unity. Hence, the wave speed is always less compared to the speed in a vacuum. The ratio of the speed of an electromagnetic wave in a vacuum to the speed in a material is known as the refractive index of the material.