The speed of an electromagnetic wave is given by the permittivity and permeability of the medium. In vacuum, this relationship is expressed in the following way: c=1ϵ0μ0√c=1ϵ0μ0, where ϵ0ϵ0 is the permittivity of free space in a vacuum, and μ0μ0 is the permeability of free space in a vacuum. These are constants of nature, and so the speed cc of the electromagnetic wave in vacuum is also a constant of nature. In a medium, the permittivity and permeability are different from their free space equivalents, and therefore cn=1ϵμ√cn=1ϵμ in the medium, where ϵϵ and μμ are the new permittivity and permeability of the medium, respectively.
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