|MadSci Network: Physics|
Yes, microwaves can pass through vacuum. Microwaves are nothing but electromagnetic waves. They do not require any medium to traverse. We do receive such signals from stellar origin, which is an exmple for their transit in vacuum. However, while guiding high intensity microwaves through wave guides, insulating gasses are used. The reason is not to provide medium for their propagation but to avoid break down of the atmosphere due to electric field generated when their path is restricted within the wave guide. See ya. Nagesh.
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