12.2.7 The Discovery of Photoelectricity

How did the Photoelectric Effect Contradict Wave Theory?

• The details of the photoelectric effect have already been covered in the Particles & Radiation section of this course
• In the photoelectric effect, incident radiation on a metal's surface causes it to emit electrons
  • However, this only happens for radiation above a certain frequency
  • If the radiation is below this threshold frequency, no matter how great the intensity, photoelectrons will not be emitted from the metal's surface

A diagram recapping the process of photoelectric emission

Light rays with a frequency above the metal's threshold frequency cause immediate emission of electrons from the surface

• This was in direct contradiction with wave theory, which predicted that an electromagnetic wave transferred energy continuously
  • According to wave theory, if low frequency radiation was aimed at the metal at a high enough intensity, then enough energy would be transferred to remove photoelectrons
• However, as soon as a radiation above the threshold frequency was shone on the metal's surface, even at low intensities, photoelectrons were immediately emitted
  • Additionally, the intensity of incident radiation only affected the number of photoelectrons emitted, but not the energy they left with
  • All of this evidence contradicted the idea that electromagnetic radiation transfers energy like a wave does

Einstein's Explanation

• In 1905 Einstein released three ground-breaking papers, one of which was a theoretical explanation of the photoelectric effect - this explanation built on Max Planck's work on black-bodies
• Einstein proposed that electromagnetic radiation was made from discrete quanta, or packets, of energy of the size:

• • Where h  is Planck's constant and f  is the frequency of the radiation
  • These quanta were called photons at a later date
  • These photons were massless in Einstein's theoretical description
• This theory was able to explain the experimental results of the photoelectric effect
• To explain why radiation below the threshold frequency didn't cause photoelectric emission: 
  • Only one photon was able to transfer its energy to only one electron
  • If hf  was not large enough to sufficiently energise an electron, the photons could not combine to energise that electron
• To explain why the energy of emitted photoelectrons increased with the frequency of incident light:
  • Photons transferred all of their energy hf to electrons
  • If this was greater than the energy needed to emit the electrons, the rest of the energy was transferred to the kinetic store of the electrons
  • If hf  was larger, more energy was left over for the kinetic store of the electrons
• Once this was all experimentally confirmed 10 years later, Einstein received a Nobel Prize in Physics