Medium

Any material through which an electromagnetic wave propagates (travels) is called a medium (plural media).

  • In optics, a medium is a material through which electromagnetic waves propagate.
  • Although electromagnetic radiation is able to propagate through a wide range of media, it is not dependent upon on any medium for propagation and travels at the speed of light through a vacuum.
  • The reason an electromagnetic wave does not need a medium to propagate through is because the only thing that is waving/oscillating is the value of the electric and magnetic fields.
  • In general terms, empty space (a vacuum) is not considered to be a medium because it does not contain matter.
  • It is the permittivity and permeability of a medium that determines how waves travel.