  We have measured the nuclear transparency of the fundamental process $\gamma$
n $\to \pi^-$ p in $^4$He. These measurements were performed at Jefferson Lab
in the photon energy range of 1.6 to 4.5 GeV and at $\theta^{\pi}_{cm}=
70^\circ$ and $90^\circ$. These measurements are the first of their kind in the
study of nuclear transparency in photoreactions. They also provide a benchmark
test of Glauber calculations based on traditional models of nuclear physics.
The transparency results suggest deviations from the traditional nuclear
physics picture. The momentum transfer dependence of the measured nuclear
transparency is consistent with Glauber calculations which include the quantum
chromodynamics phenomenon of color transparency.
