Speed of light is in general $c/n$ where $n$ is a refractive index. But for example introducing two parallel plates with very small spatial separation will perturb the energy density of vacuum reducing it in between the plates, thus effectively lowering $n$. So the rate of induction in this part of space would increase giving larger than $c$ light speed.
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This question has even been publicly studied ( since such research is highly classified) there is not too much to read on the subject and it's hard to find. See https://en.m.wikipedia.org/wiki/Scharnhorst_effect
Basically since Casmir plates lower the density of " Dirac sea particles" photons spend less time interacting with them thus spending less time as subliminal products of those interactions effectively moving at a velocity larger than c
Kugutsu-o
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You can find a related question on stackexchange here
Related to the Casimir effect , see this link.
A hand waiving, intuitive answer would be, if you change the average energy density in a region of space, you change the curvature, so you change the geodesics, but this is far from a decent answer.
I don't think there is an easy answer to this question, but I like it. Therefore, I propose to actually perform the experiment. Measure the speed of light in a direction parallel to the Casimir plates, inside a Casimir cavity.
Cristian Dumitrescu
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It Just Looks Like This Light Is Faster Than c!
If no light can travel faster than the ordinary speed of light in a vacuum, how does it seem to do so inside a Casimir Cavity. If we cannot accept that this light is really traveling faster than the ordinary speed of light in a vacuum then seemingly, we must explain how the distance becomes shorter inside the cavity. We must redefine what we mean by the actual notion of distance itself.
This new definition of distance allows for the possibility that a beam of light that passes through the the entire internal length of the Casimir Cavity actually travels a shorter internal redefined-distance than a beam of light that is traveling the entire length of the cavity, outside the cavity.
To do this, we must indulge a very dangerous question: What gives Space its properties and what is it made of? This brings us perilously close to irresponsibly resuming a belief in a material ether of some kind. Of course this is impossible since such a material ether would not be Lorentz-Invariant. Fortunately, there is an alternative. The Quantum Flux fills Space with physically real particles and energy fields and it is Lorentz-invariant.
It is not unreasonable to posit that Space itself acquires at least some of its properties from the interactions of the so-called "virtual" particles of the Zero-Point Energy Field. Maybe the property of Space that we call "a certain distance" arises because there is a certain average number and wavelength-distribution of these particles intervening between two reference points. In other words, if there are more particles between a first pair of points, then there is a longer distance between them, and if there are fewer particles between a second pair of points, then there is a shorter distance between this second pair of points.
If this is true, then when the Casimir Cavity suppresses the energy density of these Quantum Fluctuations, it is reducing the internal distances inside the Cavity. For example, the internal length of the Cavity is actually shorter than the external length of the Cavity. In other words, a beam of light that is traveling the internal length of the cavity is actually traveling at the usual Speed of Light in a Vacuum, but it is traveling a shorter distance than a comparable beam of light that travels a parallel path alongside the full external length of the cavity. (There are no wall-thickness issues since the "cavity" consists of a top plate and a bottom plate; there are no side plates in this experiment.)
(I am also not saying that the plates themselves are made shorter on the inside than on the outside; we are merely considering the possible connection between the Physical Energy Density property of Space and the geometric property of Space we call Distance.)
This explanation violates our usual presumptions about parallel distances but, seemingly, the only alternative is to accept that some light in a vacuum really does travel faster than most other light in a vacuum. That really does sound heretical!
(Paraphrasing George Orwell: All light is equal, but some light is more-equal than other light!)
