The fundamental law of quantum theory that describes the essential concept of the quanta of electromagnetic energy. Note 1: Planck's law states that the quantum of energy, E, associated with an electromagnetic field is given by E = h, where h is Planck's constant and is the frequency of the electromagnetic radiation. Note 2: Planck's constant is usually given in joule•seconds and the frequency in hertz. Thus, the quantum of energy is usually given in joules. Note 3: The product of energy and time is sometimes referred to as the elementary quantum of action. Hence, h is sometimes referred to as the elementary quantum of action.