= Brewster reflection from a tangentially moving dielectric
{c}
{title2=$|\beta_B|=n/\sqrt{1+n^2}$}
A <vacuum> ray incident normally on a <dielectric> moving parallel to its surface becomes oblique in the material rest frame, with $\sin i=|\beta|$. For a transparent nonmagnetic <dielectric>, the rest-frame <Brewster angle> satisfies $\tan i_B=n$, giving $|\beta_B|=n/\sqrt{1+n^2}$. The reflected <electric field> is perpendicular to the plane containing the ray and the velocity. A real <refractive index> $n\ne1$ is assumed; <dispersion> makes the condition implicit through the rest-frame <frequency>, and <absorption> generally removes exact extinction.
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