 A transmission medium is a material substance solid, liquid, gas, or plasma that can propagate energy waves. For example, the transmission medium for sounds is usually a gas, but solids and liquids may also act as a transmission medium for sound. The absence of a material medium in vacuum may also constitute a transmission medium for electromagnetic waves such as light and radio waves. While material substance is not required for electromagnetic waves to propagate, such waves are usually affected by the transmission media they pass through, for instance, by absorption or by reflection or refraction at the interfaces between media. The term transmission medium also refers to a technical device that employs the material substance to transmit or guide waves. Thus, an optical fiber or a copper cable is a transmission medium. Not only this but also is able to guide the transmission of networks. 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, uniformed medium or homogeneous medium, if its physical properties are unchanged at different points, isotropic medium, if its physical properties are the same in different directions. Electromagnetic radiation can be transmitted through an optical medium, such as optical fiber, or through twisted pair wires, coaxial cable, or dielectric slab waves guides. It may also pass through any physical material that is transparent to the specific wavelength, such as water, air, glass, or concrete. Sound is, by definition, the vibration of matter, so it requires a physical medium for transmission, as do other kinds of mechanical waves and heat energy. Historically, science incorporated various Aether theories 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 insulated vacuum can become conductive for electrical conduction through the presence of free electrons, holes, or ions.