How the refractive index appears in λ=u/f*n ?

[kotsoss]

Hi, I know that the wavelength is given by λ=u/f, but I saw λ=u/f*n, can someone explain how the n appers?
The speed of light is co=λοf, the speed in a medium is c=λf, there is no n in the relation.

 

[albbg]

The quantity "n" is the refractive index that express the relative speed of the light in a medium (for vacuum n=1). The same happens when an EM wave is passing through a medium having relative dielectric constant ε and relative permettivity µ. In this case the speed of the wave decreases by a factor √(ε•µ). So n=√(ε•µ). Usually µ=1 then n=√ε.

λ=c/(f*n)=c/[f*√(ε•µ)]