At the beginning of the twentieth century, the quantum nature of thermal electromagnetic radiation was a subject of intense interest. This was sparked by Max Planck’s discovery that the spectral distribution of light emerging from a thermal body could be explained only if the radiators emitting the energy occurred in discrete energy states. This induced Albert Einstein to conclude in 1905 that it was the radiation itself that was created and converted in bursts of energy; these bursts are light quanta. They were later to be called photons. The vision of such discrete energy packets travelling through space suggested to Einstein several physical consequences. In addition to rederiving Planck’s result, Einstein discussed: frequency conversion (later related to the Raman Effect), ionisation of atoms by light, and the emission of electrons from illuminated metal surfaces. The last discussion gives the theoretical description of the photoelectric effect, for which Einstein was in 1922 awarded the 1921 Nobel Prize.