For our purposes, a wave is a periodic disturbance or deformation of the medium it is in . The very existence of waves is a consequence of the medium's tendency to be at rest . Once energy is transmitted to the medium, conservation of energy requires that if the medium is to come to rest at one place, it must move the energy somewhere else. We will distinguish between several types of waves. Transverse waves disturb the medium at right angles to their direction of travel, while longitudinal waves disturb the medium in the same direction as they are travelling.
Electromagnetic waves are transverse, while sound waves are longitudinal.
The relationship between the speed of light and the permittivity and permeability was one of the first clues that light is an electromagnetic wave. The wave above has an amplitude of 2 m. It should be noted that this discussion applies to linear waves. These waves are idealizations of the waves found in nature, and are a good approximation for small amplitudes. They are called linear because they arise as solutions to linear differential equations.