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The reason the energy density of an EM wave is proportional only to the amplitude squared is because electromagnetic energy density U is:

U = (E^{2}+ B^{2})/2

in units where c = μ_{0}= ε_{0}= 1

The average value of E^{2}is proportional to E_{max}^{2}, it doesn't matter how fast E is varying, so the average energy density doesn't depend on ω.

Hurkyl

**-> http://electron9.phys.utk.edu/optics421/modules/m1/production.htm [Broken]**

The electric field is also proportional to the acceleration of the charge. The larger the acceleration, the larger is the field.

Hmmm.... If

**U ∝ E**,

^{2}**E ∝ a**and

**a ∝ ω**, how can

**U**not be dependent on

_{avg}**ω**?

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