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Intensity (physics)
Intensity (physics)
In physics, intensity is a measure of the time-averaged energy flux. The word "intensity" here is not synonymous with "strength", "amplitude", or "level", as it sometimes is in colloquial speech. For example, "the intensity of pressure" is meaningless, since the parameters of those variables do not match. To find the intensity, take the energy density (that is, the energy per unit volume) and multiply it by the velocity at which the energy is moving. The resulting vector has the units of power divided by area (i.e. watt/ m²). It is possible to define the intensity of the water coming from a garden sprinkler, but intensity is used most frequently with waves (i.e. sound or light). If the medium is damped, then the intensity drops off more quickly than the above equation suggests. Anything that can carry energy can have an intensity associated with it. For an electromagnetic wave, if E is the complex amplitude of the electric field, then the energy density of the wave is given by , and the intensity is obtained multiplying this expression by the velocity of the wave, c / n: , where n is the refractive index, c is the speed of light in vacuum and ε0 is the vacuum permittivity.
Mathematical description
If a point source is radiating energy in three dimensions and there is no energy lost to the medium, then the intensity decreases in proportion to distance from the object squared. This is due to physics and geometry. Physically, conservation of energy applies. The consequence of this is that the net power coming from the source must be constant, thus:
Alternative definitions of "intensity"
In photometry and radiometry intensity has a different meaning: it is the luminous or radiant power per unit solid angle. This can cause confusion in optics, where intensity can mean any of radiant intensity, luminous intensity or irradiance, depending on the background of the person using the term. Radiance is also sometimes called intensity, especially by astronomers and astrophysicists, and in heat transfer.
where P is the net power radiated, I is the intensity as a function of position, and dA is a differential element of a closed surface that contains the source. That P is a constant. If we integrate over a surface of uniform intensity I, for instance over a sphere centered around a point source radiating equally in all directions, the equation becomes:
See also
• Sound intensity • Magnitude (astronomy) SI photometry units
where I is the intensity at the surface of the sphere, and r is the radius of the sphere. (Asurf = 4πr2 is the expression for the surface area of a sphere). Solving for I, we get:
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From Wikipedia, the free encyclopedia
Quantity Luminous energy Luminous flux Luminous intensity Luminance Illuminance Luminous emittance Luminous efficacy SI • Photometry SI radiometry units Quantity Radiant energy Radiant flux Symbol SI unit Q Φ joule watt Abbr. J W Notes Symbol SI unit Qv F Iv Lv Ev Mv lumen second lumen (= cd·sr) candela (= lm/sr) candela per square metre lux (= lm/m2) lux (= lm/m2) lumen per watt Abbr. Notes lm·s lm cd
Intensity (physics)
units are sometimes called talbots also called luminous power an SI base unit
cd/m2 units are sometimes called "nits" lx lx Used for light incident on a surface Used for light emitted from a surface
lm/W ratio of luminous flux to radiant flux
energy radiant energy per unit time, also called radiant power power per unit solid angle power per unit solid angle per unit projected source area. called intensity in some other fields of study. power incident on a surface. sometimes confusingly called "intensity". power emitted from a surface. emitted plus reflected power leaving a surface
Radiant intensity Radiance
I L
watt per steradian watt per steradian per square metre
W·sr−1 W·sr−1·m−2
Irradiance
E, I
watt per square metre
W·m−2
Radiant exitance M / Radiant emittance Radiosity Spectral radiance J or Jλ Lλ or Lν
watt per square metre watt per square metre watt per steradian per metre3 or watt per steradian per square metre per hertz watt per metre3 or watt per square metre per hertz
W·m−2
W·m−2
W·sr−1·m−3 commonly measured in or W·sr−1·m−2·nm−1 −1·m−2·Hz−1 W·sr
Spectral irradiance
Eλ or Eν
W·m−3 or W·m−2·Hz−1
commonly measured in W·m−2·nm−1
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From Wikipedia, the free encyclopedia
Intensity (physics)
Retrieved from "http://en.wikipedia.org/wiki/Intensity_(physics)" Categories: Optics, Radiometry, Physical quantities This page was last modified on 14 May 2009, at 04:01 (UTC). All text is available under the terms of the GNU Free Documentation License. (See Copyrights for details.) Wikipedia® is a registered trademark of the Wikimedia Foundation, Inc., a U.S. registered 501(c)(3) taxdeductible nonprofit charity. Privacy policy About Wikipedia Disclaimers
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