A wave is a disturbance that propagates in a periodically repeating fashion, often transferring energy. A mechanical wave exists in a medium (which on deformation is capable of producing elastic restoring forces) through which they travel and can transfer energy from one place to another without any of the particles of the medium being displaced permanently; there is no associated mass transport. Instead, any particular point oscillates around a fixed position. However, electromagnetic radiation, and probably gravitational radiation are not mechanical waves, and can travel through a vacuum, without a medium.
Waves are characterised by crests (highs) and troughs (lows), either perpendicular (in the case of transverse waves) or parallel (in the case of longitudinal waves) to wave motion.
The medium which carries a wave
A medium that can carry a wave is classified by one or more of the following properties:
A linear medium if the amplitudes of different waves at any particular point in the medium can be added.
A bounded medium if it is finite in extent, otherwise unbounded.
A uniform medium if its physical properties are unchanged at different locations in space.
An isotropic medium if its physical properties are the same in different directions.
Examples of waves
Ocean surface waves, which are perturbations that propagate through water (see also surfing and tsunami).
Visible light, radio waves, x-rays, gamma rays, infrared rays, and ultraviolet rays make up electromagnetic radiation. In this case propagation is possible without a medium, through vacuum. These electromagnetic waves travel at about 300,000 km/s.
Sound - a mechanical wave that propagates through air, liquid or solids, and is of a frequency detected by the auditory system. Similar are seismic waves in earthquakes, of which there are the S, P and L kinds.
Gravitational waves, which are fluctuations in the gravitational field predicted by General relativity. These waves are nonlinear.
All waves have common behaviour under a number of standard situations. All waves can experience the following:
Reflection – the change of direction of waves, due to hitting a reflective surface.
Refraction – the change of direction of a wave due to them entering a new medium.
Diffraction – the spreading out of waves, for example when they travel through a small slit.
Interference – the superposition of two waves that come into contact with each other.
Dispersion – the splitting up of waves by frequency.
Rectilinear propagation – the movement of waves in straight lines.
Transverse and longitudinal waves
Transverse waves are those with vibrations perpendicular to the direction of the propagation of the wave; examples include waves on a string and electromagnetic waves. Longitudinal waves are those with vibrations parallel to the direction of the propagation of the wave; examples include most sound waves.
Ripples on the surface of a pond are actually a combination of transverse and longitudinal waves; therefore, the points on the surface follow elliptical paths.
Transverse waves can be polarized. Unpolarised waves can oscillate in any direction in the plane perpendicular to the direction of travel, while polarized waves oscillate in only one direction perpendicular to the line of travel.
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