The first indication that energy is sometimes quantized came from blackbody radiation, which is the emission of EM radiation by an object with an emissivity of 1 Another indication of energy levels being quantized in atoms and molecules comes from the lines in atomic spectra, which are the EM emissions of individual atoms and molecules

Planck recognized that the energy levels of the emitting atoms and molecules were quantized, with only the allowed values of E = n + 1 2 hf , where n is any non-negative integer (0, 1, 2, 3, …) H is Planck’s constant, whose value is h = 6 .

photoelectric effect — the phenomenon whereby some materials eject electrons when light is shined on them. quantized — the fact that certain physical entities exist only with particular discrete values and not every conceivable value. blackbody — an ideal radiator, which can radiate equally well at all wavelengths. atomic spectra — the electromagnetic emission from atoms and molecules.