Energy of Light: UNIZOR.COM - Physics4Teens - Waves - Photoelectricity

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The amount of light energy falling per unit of area per unit of time (average intensity of light) can be expressed in terms of the electric field intensity amplitude E0 as c·E0²·ε0 /2, where c is the speed of light and ε0 is the vacuum permittivity.
An important consequence from the above formula is that the density of light energy, that is the average amount of energy falling on a unit of area during a unit if time or average light intensity is proportional to a square of its amplitude.