The constituent particles of an atom are its nucleus, which does not contribute to the emission and absorption of light because it is heavy and moves very sluggishly, and the surrounding electrons, which move about quite rapidly in distinct orbits; the atom emits or absorbs a quantum of light of a definite colour when one of its electrons jumps from one orbit to another. The constituent parts of a molecule are the nuclei of the various atoms that compose the molecule and the electrons that surround each nucleus. The emission and absorption of light by a molecule are accounted for by its possible modes of rotation, by the possible modes of oscillation of its atomic nuclei, and by the periodic motions of its electrons in their various orbits. Whenever the mode of vibration or rotation of a molecule changes, changes also occur in its electronic motions, and the light of a definite colour either is absorbed or is emitted. Thus, if the wavelengths of the photons that are emitted by a molecule or an atom can be measured, considerable information can be deduced about the structure of the atom or molecule and about the possible modes of periodic motion of its constituent parts.

Continuous Spectrum
A solid object that is heated to incandescence, or by a liquid or a very dense gas emits the simplest form of spectrum, called a continuous spectrum. Such a spectrum contains no lines because light of all colours is present in it, and the colours blend continuously into one another, forming a rainbow like pattern. A continuous spectrum can be analysed only by spectrophotometric methods. In the case of an ideal emitter, a blackbody, the intensities of the colours within the spectrum depend only on the temperature. The German physicist Wilhelm Wien and the Austrian physicists Ludwig Boltzmann and Josef Stefan discovered two of the laws relating to the distribution of energy in a continuous spectrum about 1890. The Stefan-Boltzmann law states that the total energy radiated per second by a blackbody is proportional to the fourth power of the absolute temperature; Wien's displacement law states that as the temperature is raised, the spectrum of blackbody radiation is shifted toward the higher frequencies in direct proportion to the absolute temperature. In 1900 Planck discovered the third and most important law describing the distribution of energy among the various wavelengths radiated by a blackbody. In order to derive a law that interpreted his experimental findings, Planck argued that the thermodynamic properties of the thermal radiation emitted by matter must be the same regardless of the emission mechanism, and regardless of assumptions about the nature of atoms.