= Image charge at a dielectric interface
{title2=$q'=q(\epsilon_1-\epsilon_2)/(\epsilon_1+\epsilon_2)$}
For a point <electric charge> $q$ in a homogeneous medium of <permittivity> $\epsilon_1$ beside a plane interface with <permittivity> $\epsilon_2$, its potential on the source side is reproduced by a mirrored <image charge> $q'=q(\epsilon_1-\epsilon_2)/(\epsilon_1+\epsilon_2)$. The interface has no free surface charge; continuity of tangential <electric field> and normal <electric displacement field> fixes this coefficient.
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