# What is a wave? Types of waves

A wave is a disturbance that travels through space carrying energy but not matter. Waves can be classified into different categories depending on the medium (or lack thereof) through which they transfer the energy, their direction of oscillation, their shape, etc.

##### Material medium

Mechanical waves: are waves which propagate through a material medium; that is, they require a medium to transport their energy and therefore cannot propagate through vacuum. Some examples of mechanical waves are sound waves, the waves created when an object is thrown into a pond or the waves propagating along a rope.

Some animals have developed adaptations to detect through their skin tiny disturbances traveling underwater which allow them to locate their prey.

Electromagnetic waves: are waves which can propagate through vacuum and therefore do not require a medium to transport their energy. Some examples of electromagnetic waves are light, microwaves or radio waves.

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##### Direction of energy transport

Transverse waves: are waves whose oscillations are perpendicular to the direction of energy transport. Electromagnetic waves as well as certain types of waves propagating along a rope are examples of transverse waves.

Longitudinal waves: are waves whose oscillations are parallel to the direction the energy travels. Sound waves travelling through a pipe and certain types of seismic waves are examples of longitudinal waves.

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##### Dimensions

Waves can travel in one dimension (along a rope for instance), in two dimensions (waves travelling through the surface of a pond) or in three dimensions, for example the waves produced by an orchestra. When the medium through which the wave travels is homogeneous, the wave is called spherical wave.
These three types of waves are respectively called one-dimensional, two-dimensional and three-dimensional waves.

Plane wave

At great distances from the source, a small part of a spherical wavefront can be approximated by a plane, as shown in the figure below. In this case we have a plane wave. The mathematical description of a plane wave is the same as that of a one-dimensional harmonic wave.