Electromagnetic waves can travel in the vacuum as well as in matter. For example light, which is an electromagnetic wave, can travel through air, water, or glass.

Consider the electromagnetic wave passing through a dielectric medium. In such a case, Maxwell's equations get modified. In Ampere's law, ε₀ , the dielectric permittivity of free space is replaced with ε, the permittivity of dielectric. Also, the vacuum permeability μ₀ is replaced by the permeability of the medium, μ.

Furthermore, the constant speed, c is replaced with a reduced velocity, v in the expression for the relationships between electric and magnetic fields derived using Faraday's and Ampere's law. By comparing those two relations, the speed of an electromagnetic wave in a dielectric can be obtained.

For most dielectrics, the relative permeability κ_m is nearly equal to unity, excluding the insulating ferromagnetic materials. Since the dielectric constant κ is always greater than unity, the speed of electromagnetic waves in a dielectric is always less than the speed in the vacuum.

In Optics, the ratio of the speed in the vacuum to the speed in a material is known as the refractive index of the material. The dielectric constant is a function of frequency. For instance, dielectric constant for water is 80.4 for steady fields but it is approximately 1.8 in the frequency range of visible light.