= Solution
{c}
In a <local inertial frame> in <vacuum>, <gravitational waves> travel at $c$ in <general relativity>; there is no ordinary material <index of refraction> analogous to a transparent dielectric. Ordinary matter couples so weakly that laboratory prisms for <gravitational waves> are implausible. Propagation through a varying background <gravitational field> nevertheless changes ray directions and travel times, giving <gravitational lensing> and <Shapiro time delay>. In weak static coordinates $ds^2\simeq-(1+2\Phi_N/c^2)c^2dt^2+(1-2\Phi_N/c^2)d\mathbf x^2$, the null condition gives an effective coordinate <index of refraction> $n_{\rm eff}\simeq1-2\Phi_N/c^2$. This useful description depends on coordinates and does not change the local propagation <speed>. Thus astronomical gravitational refraction is plausible, while a substantial everyday material effect is not.
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