Definition of Intensity of Radiation

[Greg Bernhardt]

Definition/Summary

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 ($Wm^{-2}$)

Equations

For a surface perpendicular to the radiation:

$I\ =\ P/area$

For a general surface, with normal $\hat{\mathbf{n}}$:

$$P\ =\ \int_S \mathbf{I}\cdot\hat{\mathbf{n}}\,dS$$

Extended explanation

Power per area = energy density times velocity:

Power = energy per time (measured in $W\ =\ Js^{-1}$).

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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[APhysicist]

Thanks for the explanation on intensity. That makes it much clearer.