Photoelectric Effect

Gold Leaf Electroscope Experiment

The gold leaf electroscope experiment proves that light has particle-like properties. It involves the device shown below: two thin strips of gold leaf connected to a negatively charged metal plate. Since the two gold strips will be both negatively charged, they will repel each other, and a non-fixed gold leaf will rise up. The result from shining visible and ultraviolet light on the metal plate proves that light is not simply a wave, and proves the photoelectric effect.


Wave Theory Contradiction

Using the theory of light being a wave, we can expect that shining light of any wavelength onto the metal plate would cause the gold leaf to fall. However, this is not what happens - the gold leaf does not fall under even the brightest visible light sources. Electrons were only released from the electronscope when high-frequency light was used, e.g. ultraviolet light.



This phenomenon can be explained by the following:

  1. The energy of an electromagnetic wave is directly proportional to its frequency.
  2. Electromagnetic waves travel in discrete "packets", called photons.
  3. Metals have a minimum energy threshold required per discrete photon to release electrons.

This minimum energy threshold is known as the work function.

Confusing names!

Despite its name, a work function is not a function - it is a property of metals, similar to atomic number or atomic mass for example.


Energy of a Photon Formula

Variable Key

  • E is the energy of a photon, in joules.
  • h is the Planck constant, approximately 6.63×1034 J s.
  • f is the frequency of the wave, in hertz.

Photoelectric Effect Formula

Variable Key

  • Ek is the kinetic energy of the released photoelectron.
  • hf is the energy of a photon (from the above formula), in joules.
    • h is the Planck constant.
    • f is the frequency of the photon, in hertz.
  • hfo is the work function.
    • h is the Planck constant.
    • fo is the threshold frequency, in hertz.