If the charges are situated in a medium of permittivity ε, then the magnitude of the force between them will be, If the charges are situated in a medium of permittivity ε, then the magnitude of the force between them will be,              

    F_m  =  1/4∏€  X q₁q₂/r²        -------------------------------------(2)

Dividing equations 1 and 2 we get F/F_m =  € / €₀  =  € _r. This ratio is called as relative permittivity or dielectric constant.  This is equal to 1 in air. Hence the force between two charges depends on the medium in which they are situated.