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.
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