The Photoelectric Effect and the Beginnings of the Quantum Theory

• Black absorbs all wavelengths of radiation.  A perfectly black body is a perfect absorber of radiation and a perfect radiator as well.

• Planck used a statistical analysis that assumed radiant energy (this refers to all members of the electromagnetic spectrum) came in quanta (or small packets) to explain blackbody radiation.  The quantum theory is analogous to a ball on a staircase: you can be on the treads but not in between them.

• Einstein accepted the quanta and applied it to light when he explained the photoelectric effect.  The photoelectric effect occurs when light is incident on a metal surface and the metal emits electrons.

• The electron flow from the metal can be stopped by a negative potential.  This is called the stopping potential.

• Surprisingly, the stopping potential is independent of the intensity of the light.  Maximum KE of the ejected electrons is independent of the intensity as well.  Applying the wave theory, you would expect that greater intensity would increase the maximum KE of the ejected electrons.  In effect, the greater amplitude of the light waves would be shaking them free with more KE.  This is NOT what occurs.

• Based on the above, Einstein proposed his photoelectric equation  hf = KEmax + phi.

• hf is the energy of the incident light quanta (photons), the KE is that of the ejected electrons, and phi is the energy needed to free the electrons from the metal.

• Einstein’s analysis fits experiments beautifully, and is regarded today as one of the foundations of the quantum theory.