Information about Intensity
In physics, intensity is a measure of the time-averaged energy flux. 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).
In physics, the word "intensity" is not synonymous with "", "", or "", as it sometimes is in colloquial speech. For example, "the intensity of pressure" is meaningless, since the parameters of those variables do not match.
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:
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:
where I is the intensity at the surface of the sphere, and r is the radius of the sphere. (
is the expression for the surface area of a sphere). Solving for I, we get:
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 power carried by the wave is given by
:
where n is the refractive index,
is the speed of light in vacuum and 
is the electric permittivity in vacuum.
The magnitude of a mathematical object is its size: a property by which it can be larger or smaller than other objects of the same kind; in technical terms, an ordering of the class of objects to which
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In physics, the word "intensity" is not synonymous with "", "", or "", as it sometimes is in colloquial speech. For example, "the intensity of pressure" is meaningless, since the parameters of those variables do not match.
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:
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:
where I is the intensity at the surface of the sphere, and r is the radius of the sphere. (
is the expression for the surface area of a sphere). Solving for I, we get:
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 power carried by the wave is given by
,
:
,
where n is the refractive index,
is the speed of light in vacuum and is the electric permittivity in vacuum.
Photometry and radiometry
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.See also
| Quantity | Symbol | SI unit | Abbr. | Notes |
|---|---|---|---|---|
| Luminous energy | Qv | lumen second | lms | units are sometimes called talbots |
| Luminous flux | F | lumen (= cdsr) | lm | also called luminous power |
| Luminous intensity | Iv | candela (= lm/sr) | cd | an SI base unit |
| Luminance | Lv | candela per square metre | cdm–2 | units are sometimes called nits |
| Illuminance | Ev | lux (= lmm–2) | lx | Used for light on a surface |
| Luminous emittance | Mv | lux (= lmm–2) | lx | Used for light emitted from a surface |
| Luminous efficacy | lumen per watt | lm/W | ratio of luminous flux to radiant flux; maximum possible is 683.002 |
| Quantity | Symbol | SI unit | Abbr. | Notes |
|---|---|---|---|---|
| Radiant energy | Q | joule | J | energy |
| Radiant flux | F | watt | W | radiant energy per unit time, also called radiant power |
| Radiant intensity | I | watt per steradian | Wsr−1 | power per unit solid angle |
| Radiance | L | watt per steradian per square metre | Wsr−1m−2 | power per unit solid angle per unit projected source area. Sometimes confusingly called "intensity". |
| Irradiance | E | watt per square metre | Wm−2 | power incident on a surface. Sometimes confusingly called "intensity". |
| Radiant exitance / Radiant emittance | M | watt per square metre | Wm−2 | power emitted from a surface. |
| Radiosity | J or Jλ | watt per square metre | Wm−2 | emitted plus reflected power leaving a surface |
| Spectral radiance | Lλ or Lν | watt per steradian per metre3 or watt per steradian per square metre per hertz | Wsr−1m−3 or Wsr−1m−2Hz−1 | commonly measured in Wsr−1m−2nm−1 |
| Spectral irradiance | Eλ or Eν | watt per metre3 or watt per square metre per hertz | Wm−3 or Wm−2Hz−1 | commonly measured in Wm−2nm−1 |
Physics is the science of matter[1] and its motion[2][3], as well as space and time[4][5] —the science that deals with concepts such as force, energy, mass, and charge.
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Measurement is the estimation of the magnitude of some attribute of an object, such as its length or weight, relative to a unit of measuremnt. Measurement usually involves using a measuring instrument, such as a ruler or scale, which is calibrated to compare the object to some
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energy (from the Greek ενεργός, energos, "active, working")[1] is a scalar physical quantity that is a property of objects and systems of objects which is conserved by nature.
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flux, both with rigorous mathematical frameworks.
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- In the study of transport phenomena (heat transfer, mass transfer and fluid dynamics), flux is defined as the amount that flows through a unit area per unit time, the volumetric flow rate.
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velocity is defined as the rate of change of position. It is a vector physical quantity, both speed and direction are required to define it. In the SI (metric) system, it is measured in meters per second (m/s). The scalar absolute value (magnitude) of velocity is speed.
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spatial vector, or simply vector, is a concept characterized by a magnitude and a direction. A vector can be thought of as an arrow in Euclidean space, drawn from an initial point A pointing to a terminal point B.
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In physics, power (symbol: P) is the rate at which work is performed or energy is transmitted, or the amount of energy required or expended for a given unit of time.
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Area is a physical quantity expressing the size of a part of a surface. The term Surface area is the summation of the areas of the exposed sides of an object.
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Units
Units for measuring surface area include:- square metre = SI derived unit
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wave is a mode of energy transfer from one place to another, often with little or no permanent displacement of the particles of the medium (i.e. little or no associated mass transport); instead there are oscillations around almost fixed positions.
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Sound is a disturbance of mechanical energy that propagates through matter as a wave (through fluids as a compression wave, and through solids as both compression and shear waves).
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Light is electromagnetic radiation of a wavelength that is visible to the eye (visible light). In a scientific context, the word "light" is sometimes used to refer to the entire electromagnetic spectrum.
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conservation of energy states that the total amount of energy in any closed system remains constant but can be recreated, although it may change forms, e.g. friction turns kinetic energy into thermal energy.
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The refractive index (or index of refraction) of a medium is a measure for how much the speed of light (or other waves such as sound waves) is reduced inside the medium. For example, typical glass has a refractive index of 1.
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speed of light in a vacuum is an important physical constant denoted by the letter c for constant or the Latin word celeritas meaning "swiftness".[1] It is the speed of all electromagnetic radiation, including visible light, in a vacuum.
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A vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than standard atmospheric pressure. The Latin term in vacuo is used to describe an object as being in what would otherwise be a vacuum.
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Permittivity is a physical quantity that describes how an electric field affects and is affected by a dielectric medium, and is determined by the ability of a material to polarize in response to the field, and thereby reduce the total electric field inside the material.
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Photometry is the science of measurement of light, in terms of its perceived brightness to the human eye. It is distinct from radiometry, which is the science of measurement of light in terms of absolute power; rather, in photometry, the radiant power at each wavelength is
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In optics, radiometry is the field that studies the measurement of electromagnetic radiation, including visible light. Note that light is also measured using the techniques of photometry, which deal with brightness as perceived by the human eye, rather than absolute power.
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The solid angle, Ω, is the angle that an object subtends at a point. It is a measure of how big that object appears to an observer at that point. For instance, a small object nearby could subtend the same solid angle as a large object far away.
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Optics (ὀπτική appearance or look in Ancient Greek) is a branch of physics that describes the behavior and properties of light and the interaction of light with matter.
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In radiometry, radiant intensity is a measure of the intensity of electromagnetic radiation. It is defined as power per unit solid angle. The SI unit of radiant intensity is watts per steradian (W·sr-1).
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In photometry, luminous intensity is a measure of the wavelength-weighted power emitted by a light source in a particular direction, based on the luminosity function, a standardized model of the sensitivity of the human eye.
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Irradiance, radiant emittance, and radiant exitance are radiometry terms for the power of electromagnetic radiation at a surface, per unit area. "Irradiance" is used when the electromagnetic radiation is incident on the surface.
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Radiance and spectral radiance are radiometric measures that describe the amount of light that passes through or is emitted from a particular area, and falls within a given solid angle in a specified direction.
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In photometry, luminous intensity is a measure of the wavelength-weighted power emitted by a light source in a particular direction, based on the luminosity function, a standardized model of the sensitivity of the human eye.
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The sound intensity, I, (acoustic intensity) is defined as the sound power Pac per unit area A. The usual context is the noise measurement of sound intensity in the air at a listener's location.
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amplitude is a nonnegative scalar measure of a wave's magnitude of oscillation, that is, the magnitude of the maximum disturbance in the medium during one wave cycle.
Sometimes this distance is called the peak amplitude
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Sometimes this distance is called the peak amplitude
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For other senses of this word, see magnitude.
The magnitude of a mathematical object is its size: a property by which it can be larger or smaller than other objects of the same kind; in technical terms, an ordering of the class of objects to which
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Si, si, or SI may refer to (all SI unless otherwise stated):
In language:
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In language:
- One of two Italian words:
- sì (accented) for "yes"
- si
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In photometry, luminous energy is the perceived energy of light. This is sometimes also called the quantity of light.
Luminous energy is not the same as the radiant energy, the corresponding objective physical quantity.
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Luminous energy is not the same as the radiant energy, the corresponding objective physical quantity.
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