  High-frequency photons traveling in plasma exhibit a linear polarizability
that can influence the dispersion of linear plasma waves. We present a detailed
calculation of this effect for Langmuir waves as a characteristic example. Two
alternative formulations are given. In the first formulation, we calculate the
modified dispersion of Langmuir waves by solving the governing equations for
the electron fluid, where the photon contribution enters as a ponderomotive
force. In the second formulation, we provide a derivation based on the photon
polarizability. Then, the calculation of ponderomotive forces is not needed,
and the result is more general.
