Planck's law

Planck's law is the fundamental law of quantum theory that describes the essential concept of the quanta of electromagnetic energy.

Planck's law states that the energy associated with electromagnetic radiation comes in indivisible packets called quanta. Each such quantum is associated with a single photon. The size E of the quanta depends on the frequency f of the radiation according to the formula

E = hf,

where h is Planck's constant.

Planck's constant is usually given in joule·seconds and the frequency in hertz. Thus, the quantum of energy is usually given in joules.

The product of energy and time is sometimes referred to as action. Hence, h is sometimes referred to as the elementary quantum of action.

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Source: from Federal Standard 1037C