Experiments show that light comes in discrete packets, called photons .
The energy E of a photon is related to its frequency by Here h is called Plank's constant and is very tiny. Thus there are zillions of photons in an ordinary light beam. But for frequencies as in visible light or for larger frequencies, the energy of a single photon is not so small compared to energies of single atoms. The photon concept is important because photons are emitted and absorbed one photon at a time . For instance, a molecule in a vision cell in your eye can absorb one photon, but never half a photon.