Intensity (physics)
- For other uses, see Intensity (disambiguation).
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 "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.
If a point source is radiating energy in three dimensions and there is no energy lost to the medium, then the intensity drops off 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:
- <math>P = \int I\, dA</math>
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 the source is radiating uniformly, i.e. the same in all directions, and we take A to be a sphere centered on the source (so that I will be constant on its surface), the equation becomes:
- <math>P = |I| \cdot 4 \pi r^2 \,</math>
where I is the intensity at the surface of the sphere, and r is the radius of the sphere. (<math>4 \pi r^2 </math> is the expression for the surface area of a sphere). Solving for I, we get:
- <math>|I| = \frac{P}{4 \pi r^2}</math>
Anything that can carry energy can have an intensity associated with it.
If the medium is damped (i.e. both sound and light in air slowly lose energy), then the intensity drops off more quickly than the above equation suggests.
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 | lm·s | units are sometimes called Talbots |
| Luminous flux | F | lumen (= cd·sr) | lm | also called luminous power |
| Luminous intensity | Iv | candela (= lm/sr) | cd | an SI base unit |
| Luminance | Lv | candela per square metre | cd/m2 | units are sometimes called nits |
| Illuminance | Ev | lux (= lm/m2) | lx | Used for light incident on a surface |
| Luminous emittance | Mv | lux (= lm/m2) | 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 | Φ | watt | W | radiant energy per unit time, also called radiant power |
| Radiant intensity | I | watt per steradian | W·sr−1 | power per unit solid angle |
| Radiance | L | watt per steradian per square metre | W·sr−1·m−2 | power per unit solid angle per unit projected source area. Sometimes confusingly called "intensity". |
| Irradiance | E | watt per square metre | W·m−2 | power incident on a surface. Sometimes confusingly called "intensity". |
| Radiant exitance / Radiant emittance | M | watt per square metre | W·m−2 | power emitted from a surface. Sometimes confusingly called "intensity". |
| Spectral radiance | Lλ or Lν | watt per steradian per metre3 or watt per steradian per square metre per hertz | W·sr−1·m−3 or W·sr−1·m−2·Hz−1 | commonly measured in W·sr−1·m−2·nm−1 |
| Spectral irradiance | Eλ or Eν | watt per metre3 or watt per square metre per hertz | W·m−3 or W·m−2·Hz−1 | commonly measured in W·m−2·nm−1 |
Categories
Optics | Radiometry | Physical quantity