A medium is any substance that allows the propagation of electromagnetic waves.

  • In optics, a medium is any substance that can modify the path of light.
  • Electromagnetic radiation can propagate through different media, but it can also propagate through a vacuum.
  • Electromagnetic waves do not require a medium for propagation because they are the result of the interaction between electric and magnetic fields, and the wave itself is a self-sustaining process that propagates through space.
  • In general terms, a vacuum is not considered to be a medium because it has no material substance.
  • The permittivity and permeability of a substance determine the speed at which electromagnetic waves propagate through that substance.