Intensitetsrelation

Intensitetsrelation is a term used to describe how intensity, the flow of energy per unit area or per unit solid angle, relates to other physical quantities within a system. The concept appears across fields such as optics, acoustics and radiative transfer, and the specific relation depends on the context and the radiometric quantities involved (for example irradiance, radiance and radiant intensity).

In optics, the intensity of an electromagnetic wave is related to the time-averaged Poynting vector. For a monochromatic plane wave, the average intensity I is proportional to the square of the electric field amplitude: I ∝ E0^2. It is also connected to photon flux, with the photon arrival rate per unit area Φ = I/(hν), where ν is the frequency and h Planck’s constant. This links power transport to measurable signals such as detector responses.

In acoustics, sound intensity I is defined as the product of acoustic pressure p and particle velocity u, I = p·u, and can also be written as I = p^2/(ρc) where ρ is density and c is the speed of sound. This expresses how acoustic energy propagates through a medium.

In radiative transfer, the directional intensity along a path obeys the radiative transfer equation: dIν/ds = -αν Iν + jν, where ν is frequency, αν is the absorption coefficient and jν is the emission coefficient. This captures how intensity changes due to absorption and emission.

Units and quantities in play include radiant intensity (W per steradian), irradiance (W per square meter) and radiance (W per (steradian square meter)). Intensitetsrelation thus denotes the guiding mathematical links between these quantities across different physical contexts.