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| {{Unreferenced|date=December 2007}}
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| A '''point source''' is a single identifiable ''localised'' source of something. A point source has [[negligible]] extent, distinguishing it from other source geometries. Sources are called point sources because in mathematical modeling, these sources can usually be approximated as a mathematical [[Point (geometry)|point]] to simplify analysis.
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| The actual source need not be physically small, if its size is negligible relative to other length scales in the problem. For example, in [[astronomy]], [[star]]s are routinely treated as point sources, even though they are in actuality much larger than the [[Earth]].
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| In [[Three-dimensional space|three dimensions]], the density of something leaving a point source decreases in proportion to the [[inverse square]] of the [[distance]] from the source, if the distribution is [[isotropic]], and there is no [[absorption (electromagnetic radiation)|absorption]] or other loss.
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| ==Mathematics==
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| In mathematics, a '''point source''' is a [[Mathematical singularity|singularity]] from which [[flux]] or flow is emanating. Although singularities such as this do not exist in the observable universe, <!--NB black holes are not in the observable universe as they have an event horizon --> mathematical point sources are often used as approximations to reality in [[physics]] and other fields.
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| ==Light==
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| Generally a [[light source|source of light]] can be considered a point source if the resolution of the imaging instrument is too low to resolve its apparent size.
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| Mathematically an object may be considered a point source if its angular size, <math>\theta</math>, is much smaller than the resolving power of the telescope:<br />
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| <math>\theta << \lambda / D</math>,<br />
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| where <math>\lambda</math> is the wavelength of light and <math>D</math> is the telescope diameter.
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| Examples:
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| * Light from a distant star seen through a small telescope
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| * Light passing through a [[pinhole]] or other small [[aperture]], viewed from a distance much greater than the size of the hole
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| * Light from a [[street light]] in a large-scale study of [[light pollution]] or street [[Lighting|illumination]]
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| ==Electromagnetic radiation==
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| [[Radio wave]] sources which are smaller than one radio [[wavelength]] are also generally treated as point sources. Radio emissions generated by a fixed electrical circuit are usually [[Polarization (waves)|polarized]], producing [[anisotropic]] radiation. If the propagating medium is lossless, however, the radiant power in the radio waves at a given distance will still vary as the inverse square of the distance if the angle remains constant to the source polarization.
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| [[Gamma ray]] and [[X-ray]] sources may be treated as a point source if sufficiently small. [[Radiological contamination]] and nuclear sources are often point sources. This has significance in [[health physics]] and [[radiation protection]].
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| Examples:
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| *[[Antenna (radio)|Radio antennas]] are often smaller than one wavelength, even though they are many metres across
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| *[[Pulsars]] are treated as point sources when observed using [[radio telescopes]]
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| *In nuclear physics, a "hot spot" is a point source of [[radiation]]
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| ==Sound==
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| [[Sound]] is an oscillating [[pressure]] wave. As the pressure oscillates up and down, an audio point source acts in turn as a fluid point source and then a fluid point sink. (Such an object does not exist physically, but is often a good simplified model for calculations.)
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| Examples:
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| * [[Seismology|Seismic vibration]] from a localised seismic experiment searching for oil
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| * [[Noise pollution]] from a [[jet engine]] in a large-scale study of noise pollution
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| * A [[loudspeaker]] may be considered as a point source in a study of the [[acoustics]] of [[airport]] announcements
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| ==Heat==
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| [[Image:Nagasakibomb.jpg|thumb|right|A [[mushroom cloud]] as an example of a thermal plume. A nuclear explosion can be treated as a thermal point source in large-scale atmospheric simulations.]]
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| In [[vacuum]], heat escapes as [[radiation]] isotropically. If the source remains stationary in a [[compressible fluid]] such as [[air]], flow patterns can form around the source due to [[convection]], leading to an [[anisotropy|anisotropic]] pattern of heat loss. The most common form of anisotropy is the formation of a thermal [[plume (hydrodynamics)|plume]] above the heat source.
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| Examples:
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| *Geological hotspots on the surface of the Earth which lie at the tops of thermal plumes rising from deep inside the Earth
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| *Plumes of heat studied in [[thermal pollution]] tracking.
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| ==Fluid== | |
| Fluid point sources are commonly used in [[fluid dynamics]] and [[aerodynamics]]. A point source of fluid is the inverse of a fluid point sink (a point where fluid is removed). Whereas fluid sinks exhibit complex rapidly changing behaviour such as is seen in [[vortex|vortices]] (for example water running into a plug-hole or [[tornado]]es generated at points where air is rising), fluid sources generally produce simple flow patterns, with stationary isotropic point sources generating an expanding sphere of new fluid. If the fluid is moving (such as wind in air or currents in water) a [[plume (hydrodynamics)|plume]] is generated from the point source.
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| Examples:
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| * [[Air pollution]] from a [[power plant]] [[flue gas stack]] in a large scale analysis of air pollution
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| * [[Water pollution]] from an [[oil refinery]] [[wastewater]] discharge outlet in a large scale analysis of water pollution
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| * Gas escaping from a pressurised pipe in a laboratory
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| * Smoke is often released from point sources in a [[wind tunnel]] in order to create a [[plume (hydrodynamics)|plume]] of smoke which highlights the flow of the wind over an object
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| * Smoke from a localised chemical fire can be blown in the wind to form a [[plume (hydrodynamics)|plume]] of pollution
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| ==Pollution==
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| {{main|Point source (pollution)}}
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| Sources of various types of pollution are often considered as point sources in large-scale studies of pollution.
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| == See also ==
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| * [[Line source]]
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| * [[Dirac delta function]]
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| [[Category:Experimental physics]]
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| [[Category:Mathematical physics]]
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The writer is known as Wilber Pegues. Mississippi is where her home is but her spouse wants them to move. He is an purchase clerk and it's something he truly enjoy. What I love doing is football but I don't have the time lately.
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