I was on Science fiction and fantasy and there was a question on wether or not a lightsaber can cast a shadow, but that would depend on wether or not plasma can cast a shadow.
I became curios and I would like to further my knowledge on the Physics of plasma.
I know little about the physics of light, and much less plasma, so add as much detail to your answer as you can for my learning sake.
It depends on whether the light sabre is "optically thick" to light. For a fully ionised plasma and a continuum light source, you would be relying on Thomson scattering from free electrons to provide opacity. The cross-section per electron is $\sigma = 6.6\times 10^{-29}$ m$^2$ and independent of wavelength.
The mean free path of light in the plasma is $(n\sigma)^{-1}$, where $n$ is the electron number density. This then has to be smaller than the width of the sabre "beam" if it is to block the light and cast a shadow. If we imagine the "beam" width is about 5cm, then $ n> (0.05\sigma)^{-1} = 3\times 10^{29}$ m$^{-3}$.
If the plasma is pure ionised hydrogen, then it would have a density of 500 kg/m$^3$, so I guess this is just about conceivable/portable.
Of course you could cheat. For example if you take a partially ionised plasma and then only illuminate it with a lamp that emits photons at the discrete resonant frequency corresponding to a particular atomic/ionic transition in the plasma, then it could be much sparser and cast a shadow because resonant cross-sections are much greater than the Thomson scattering cross-section.
Take a near-colourless Bunsen burner flame.
If you add some sodium (eg a spray of salt water), you will see the typical emission spectrum of excited sodium atoms. But the flame will also contain large amounts of ground state sodium atoms.
Now turn on a sodium vapour lamp (low pressure). Again, you will see the same sodium atomic emission spectrum.
But you will also see the flickering black shadow cast as the cool sodium in and above the flame absorbs the light from the sodium lamp.
I found this experiment with plasma : A laboratory plasma experiment for studying magnetic dynamics of accretion discs and jets
. A high-speed multiple-frame CCD camera reveals images of the formation and helical instability of a collimated plasma,
Images of plasma evolution (shot 1210; peak αgun≈66 m-1)in which a plasma column forms and persists for many Alfvén transit times, illustrating the magnetic topology required for an astrophysical jet.
They are using hydrogen and are studying the magnetic control of plasma. One can see thicker lines.
I tried seeing the shadow of a candle flame(though not a plasma, still it is an emitter of light) using a flashlight: there was a mist around the wick shadow, the light interacting with the burning molecules of wax.
If the imaginative lightsaber is molded by magnetic fields it looks as if there should be a shadow in a strong light source.
