What is a photon?

   A photon is the elementary bundle or packet of electromagnetic energy absorbed or emitted by an atom or a molecule. Electromagnetic energy travels as a set of waves. But the emission or absorption of the electromagnetic energy occurs only in units called quanta. A quantum of energy is the energy carried by a single photon.
   When an atom loses energy by passing from one state to another, it must satisfy the principle of conservation of energy. The energy lost is taken up by some other atom or given off in some form of radiation.
   Niels Bohr, a Danish physicist, adopted the energy packet idea from Max Planck, a professor at the University of Berlin. There were certain phenomena, such as the photoelectric effect, which indicated that light did not always behave as a wave. The packet theory states that energy can be emitted or absorbed only in small but definite amounts, called quanta or photons.