Light behaves in some ways like a wave, and other other ways like a particle. Generally, the more energetic it is, the more it seems like a particle; but the longer the wave length the more it seems like a wave.

There are some intriguing experiments, the results of which cannot be understood except by recognizing that light is both a wave and a particle. One of these is the single photon/double slit experiment. Light is directed through two slits, and we find an interference pattern. This is a wave-like phenomenon, and most easily explained in terms of a wave passing through both slits, and interfering with itself to give the pattern. But we can make it particle-like as well, by using light of such low intensity that there can only be one photon in transit at any time, or by using photoelectric detectors to see where the light arrives. Such detectors reply on particle-like properties of light.

Such a set up still shows an interference pattern. Somehow, even if considered one photon at a time, the photon path depends on both the slits. Sometimes people speak of the photon passing through both slits at once. This is an instance of wave particle duality, which is at the heart of quantum physics. Quantum physics is (IMO) one of the hardest parts of physics to grasp. Even if you can crunch all the numbers and formulae correctly, finding an intuition for what you are calculating is difficult and tends to defy some very basic assumptions we have about how the world works.

Returning to the photon with a wavelength of 50,000 km. Such a photon does not have a clearly identified location. Its trajectory will be impacted by many electrons over a wide area, in much the same way as a photon trajectory may be impacted by having a range of alternative slits that interfere with each other. Put another way, such a photon does not "see" individual particles in the plasma. It sees the combined fields, and interacts with the plasma as a whole.

However, if the photon is highly energetic, then it is more localised. A visible light photon, with a wavelength of 500 nm, can only "see" one electron at a time in the rarefied intergalactic plasma.

In reading the notes on plasma physics that have been cited in the thread, and also further notes from the same sites and sources, the analysis of index of refraction and speed of propagation for light in a plasma is treated with wave analysis. The more energetic the light, the less propagation speed is affected. One of the terms in the formulae is sqrt(f^2-p^2), where f is the frequency of the electromagnetic wave and p is the plasma frequency. This becomes imaginary with f is less than the plasma frequency, which corresponds to damping of the wave. But above the plasma frequency, and you have transmission with reduced velocity but no energy loss. For a frequency far in excess of the plasma frequency, the difference in speed of propagation is negligible, and there is effectively no refraction.

Cheers -- Sylas

PS. The stuff on plasma is based on what I have been learning just in the last week. Don't take it as expert opinion. Comments or corrections welcome.