Intensity (physics)

The intensity is an energy quantity. The intensity commonly referred to in physics, the energy per unit time per unit area, that is, a surface power density. There is equal to the energy density ( energy per unit volume ) multiplied by the rate at which the energy moves. According to this definition, anything that transports energy, have an associated intensity. The term is used mostly in wave phenomena such as sound or light. An example is the sound intensity.

As intensity is outside physics also "strength ", " power ", " amplitude ", or " level " means, but this is not equivalent to each.

One must distinguish between the rather broad definition of the intensity in physics and narrow definition in photometry. In photometry, we distinguish among three different sizes that are often confused with the general definition in physics:

• Irradiance: Radiant power per unit area in watts per square meter
• Intensity of radiation: radiation power per unit solid angle in watts per steradian
• Aperture: radiation intensity weighted by the Empfindlichekeit of the eye in candelas

The first two are interpreted as the intensity in the physical sense. The chosen definition is apparent in the driving physics to the unit.

Irradiance ( intensity) in the wave theory

The intensity is sometimes erroneously mistaken for the definition of the irradiance, which is defined as

The Poynting vector, denotes the temporal averaging.

The equation

With an energy density and as a group rate applies only to transparent media. ( A transparent medium is a material with no dispersion. )

The irradiance can be expressed as a flow of energy or power density as power per area (for example, so in the unit ).

In the wave theory, the intensity is proportional to the square of the amplitude A of the wave:

Intensity of a monochromatic plane electromagnetic wave

With a monochromatic plane electromagnetic wave in vacuum resulting from the general definition of the following explicit expression for the intensity:

It is the speed of light, the electric field constant and the maximum amplitude of the electric field of the wave.

In linear dielectric media applies:

Having a refractive index, wherein the susceptibility of the medium.

Intensity of a point source

Exuding a point source, such as a sound source of energy in three dimensions and there is no loss of energy, the intensity falls with the distance r from the object with from:

This has a simple geometric reason. Is the total radiated power

Said intensity referred to as a function of position, and since the surface differential of a closed surface which surrounds the sound source. If the source uniformly in all directions ( isotropic), and the area A is a sphere of radius r encloses, in the middle is the sound source, the equation simplifies to

After I (r ) obtained resolved

This relationship states that the intensity I reciprocal square decreases with the distance r from the source according to the law of distance, So with 1/r2.

In this form corresponds to the photometric irradiance. Looking at the part of the intensity of the lights on a sphere of radius 1m an area of ​​one square meter is obtained from this the photometric radiation intensity which is independent of r for spherical waves.

Influence of a medium

If the medium attenuates ( absorbs ), loses the wave energy, which is converted into heat energy, for example. Assuming that the decrease in intensity is proportional to the existing intensity at each location, then, analogous to the decay law is an exponential curve, the so-called Lambert -Beer law:

With increasing propagation of the wave in the medium thus decreases its intensity decreases exponentially. In this case, the absorption coefficient describes the material properties of the medium traversed.