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,              
    Fm  =  1/4∏€  X q1q2/r2        -------------------------------------(2)
Dividing equations 1 and 2 we get F/Fm =  € / €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.