- Transmission medium
A transmission medium"' (plural "transmission media") is a material substance (
solid, liquidor gas) which can propagate energy waves. For example, the transmission medium for soundreceived by the ears is usually air, but solids and liquids may also act as transmission media for sound.
The absence of a material medium (the
vacuumof empty space) can also be thought of as a transmission medium for electromagnetic waves such as lightand radio waves. While material substance is not required for electromagnetic waves to propagate, such waves are usually affected by the transmission media through which they pass, for instance by absorption or by reflectionor refractionat the interfaces between media.
The term transmission medium can also refer to the technical device which employs the material substance to transmit or guide the waves. Thus an optical fiber or a copper cable can be referred to as a transmission medium.
A transmission medium can be classified as a:
*"Linear medium", if different waves at any particular point in the medium can be superposed;
*"Bounded medium", if it is finite in extent, otherwise "unbounded medium";
*"Uniform medium", if its physical properties are unchanged at different points;
*"Isotropic medium", if its physical properties are the same in different directions.
Electromagnetic radiationcan be transmitted through an optical media, such as optical fiber, or through twisted pairwires, coaxial cable, or dielectric-slab waveguides. It may also pass through any physical material which is transparent to the specific wavelength, such as water, air, glass, or concrete. Soundis, by definition, the vibration of matter, so it requires a physical medium for transmission, as does other kinds of mechanical waves and heatenergy. Historically, various aether theorieswere used in science and thought to be necessary to explain the transmission medium. However, it is now known that electromagnetic waves do not require a physical transmission medium, and so can travel through the " vacuum" of free space. Regions of the insulative vacuum can become conductivefor electrical conductionthrough the presence of free electrons, holes, or ions.
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