If I draw a control volume around a single light wave traveling through empty space, is entropy increasing as it goes? How is the degradation of the quality of energy manifested?
Answer
No, light propagating through space doesn't increase entropy. It's generally considered to be a reversible process, for the simple reason that a suitably curved mirror can be used to restore the initial state. Instead, entropy is produced when light is absorbed or emitted from any surface or volume.
In the ideal case of a black body, the entropy produced by emission is $\frac{4}{3}\frac{J}{T}$, where $T$ is the temperature of the body and $J$ is the net rate of emission, in $\mathrm{Js^{-1}}$. This entropy is associated with the light radiating away from the black body, and its entropy doesn't change until it is absorbed by another body.
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