Anything that can transmit energy can have an intensity associated with it. For a monochromatic propagating electromagnetic wave, such as a plane wave or a Gaussian beam, if E is the complex amplitude of the electric field, then the time-averaged energy density of the wave, travelling in a non-magnetic material, is given by: ⟨ U ⟩ = n 2 ε ε 0 2 | E | 2 , and the local intensity is obtained by multiplying this expression by the wave velocity, ⁠ c n : ⁠ I = c n ε ε 0 2 | E | 2 , where