Intensity of a sinusoidal EM plane wave in vacuum (I = ⟨S⟩ = ½ c ε₀ E₀² (vacuum plane wave))

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I = ⟨S⟩ = ½ c ε₀ E₀² (vacuum plane wave)

What each symbol means

SymbolWhat it stands for
IIntensity — average power per unit area (W/m²)
cSpeed of light in vacuum
E₀Peak electric field amplitude (V/m)
ε₀Permittivity of free space

When to use this

Sinusoidal plane wave in vacuum; time-averaged Poynting magnitude.