Einstein's interpretation

Planck proposed that electromagnetic radiation was not emitted and absorbed continuously, but was emitted and absorbed in little bundles of energy called quanta.

According to Planck, the energy of a quantum was proportional to the frequency of the wave.

Einstein accepted the notion of quanta and described light as a stream of photons (quanta), each travelling with the velocity of light. Einstein suggested that when a photon collides with an electron, the transfer of energy is an “all or nothing” process, i.e. the electron gets all the photon’s energy or none of it. If a photon gives all its energy to an electron, then it simply drops out of existence. The energy acquired by an electron may enable it to escape from the metal.

If a photon is to liberate an electron from a metal by the photoelectric effect, its energy must exceed the work function of the metal. Any excess energy which the photon has will appear as K.E. of the electron which escapes.

Summary:

Einstein's Photoelectric Equation:  

Theory
Einstein's Solution
Photon

Glossary

Light comes in little chunks of energy, called quanta or photons. 

The energy of a photon is proportional to the light frequency:
E = hf.

The work function Φ is the amount of energy needed to liberate one electron from the metal's surface.