Transcript: About a hundred and fifty years ago scientists were faced with a profound mystery in their understanding of matter and radiation. Astronomers had observed sharp spectral features in the emission of nebulae and stars. Similarly hot gases in the laboratory showed sharp spectral features in the spectra, and since a spectrum is a map of energy this meant that there were particular energies of transition in atoms. This has no way of being understood in classical physics because in classical physics atoms can have any energy. The mystery was solved by the German physicist Max Planck who developed the quantum theory of radiation and matter. In the quantum theory atoms cannot have any energy but only particular energies. Just as matter has fundamental units, such as protons and electrons, so radiation has a fundamental indivisible unit called a quantum. The quanta of light and other forms of electromagnetic radiation are called photons.