  Newly recognized effects of refractive scattering in the ionized interstellar
medium have broad implications for very long baseline interferometry (VLBI) at
extreme angular resolutions. Building upon work by Blandford & Narayan (1985),
we present a simplified, geometrical optics framework, which enables rapid,
semi-analytic estimates of refractive scattering effects. We show that these
estimates exactly reproduce previous results based on a more rigorous
statistical formulation. We then derive new expressions for the
scattering-induced fluctuations of VLBI observables such as closure phase, and
we demonstrate how to calculate the fluctuations for arbitrary quantities of
interest using a Monte Carlo technique.
