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.

|500

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.

|500

Explanation

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.

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.

Formulae

Energy of a Photon Formula

E = h f

Variable Key

$E$ is the energy of a photon, in joules.
$h$ is the Planck constant, approximately $6.63 \times 10^{- 34} J s$ .
$f$ is the frequency of the wave, in hertz.

Photoelectric Effect Formula

E_{k} = h f - h f_{o}

Variable Key

$E_{k}$ is the kinetic energy of the released photoelectron.
$h f$ 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.
$h f_{o}$ is the work function.
- $h$ is the Planck constant.
- $f_{o}$ is the threshold frequency, in hertz.