<Li> Uniform medium or homogeneous medium, if its physical properties are unchanged at different points; </Li> <Li> Isotropic medium, if its physical properties are the same in different directions . </Li> <P> Electromagnetic radiation can be transmitted through an optical medium, such as optical fiber, or through twisted pair wires, coaxial cable, or dielectric - slab waveguides . 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 insulative vacuum can become conductive for electrical conduction through the presence of free electrons, holes, or ions . </P> <P> A physical medium in data communications is the transmission path over which a signal propagates . </P>

Which type of wave requires a medium for transmission