Unlike the case of the metal, the image charge q ′ is not exactly opposite to the real charge: q ′ = ε ε 1 − − ε ε 2 ε ε 1 + ε ε 2 q . It may not even have the same sign, if the charge is placed inside the stronger dielectric material (charges are repelled away from regions of lower dielectric constant). This can be seen from the formula.