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:
- The energy of an electromagnetic wave is directly proportional to its frequency.
- Electromagnetic waves travel in discrete "packets", called photons.
- Metals have a minimum energy threshold required per discrete photon to release electrons.
This minimum energy threshold is known as the work function.
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
is the energy of a photon, in joules. is the Planck constant, approximately . is the frequency of the wave, in hertz.
Photoelectric Effect Formula
is the kinetic energy of the released photoelectron. is the energy of a photon (from the above formula), in joules. is the Planck constant. is the frequency of the photon, in hertz.
is the work function. is the Planck constant. is the threshold frequency, in hertz.