QUOTE : The “quantum theory” came into being as an effort to cure the wave theory of light of these defects. It has been completely successful. It has shewn that Newton was not wholly wrong in regarding light as corpuscular, for it has proved that a beam of light may be regarded as broken up into discrete units, called “light-quanta” or “photons,” with almost the definiteness with which a shower of rain may be broken up into drops of water, a shower of bullets into separate pieces of lead, or a gas into separate molecules. At the same time, the light does not lose its undulatory character. Each little parcel of light has a definite quantity, of the nature of a length, associated with it. We call this its “wave-length,” because when the light in question is passed through a prism, it behaves exactly as waves of this particular length of wave would do. Light of long wavelength is made up of small parcels, and vice-versa, the amount of energy in each parcel being inversely proportional to this wave-length, so that we can always calculate the energy of a photon from its wave-length, and vice-versa.