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Intensity of radiation = power per area perpendicular to the radiation. It is a vector, parallel to the radiation.

Intensity of radiation = energy density times velocity (therefore sometimes called energy flux).

It is measured in watts per square metre ([itex]Wm^{-2}[/itex])

Equations

For a surface perpendicular to the radiation:

[itex]I\ =\ P/area[/itex]

For a general surface, with normal [itex]\hat{\mathbf{n}}[/itex]:

[tex]P\ =\ \int_S \mathbf{I}\cdot\hat{\mathbf{n}}\,dS[/tex]

Extended explanation

Power per area = energy density times velocity:

Power = energy per time (measured in [itex]W\ =\ Js^{-1}[/itex]).

Power per area = energy per area per time = energy density (ie, per volume) times velocity.

So intensity = power per area = energy density times velocity.

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# What is intensity

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