Photoelectric effect

The study of light is at the heart of the greatest scientific discoveries. For centuries, man studied the effects of light without understanding its causes and even less its nature. How do we see it? How does light travel and in what medium? Does it spread instantly? If not, how fast is light? Is it a wave or is it a particle? It is precisely this last question that the photoelectric effect experiment answers.

• 1670s: Huygens explains the laws of diffraction according to a wave model.
• ​1700s: Newton breaks down white light and claims that light is a stream of moving particles.
• 1840s: Becquerel and Faraday discover interactions between matter and light.
• 1860s: Maxwell discovers the electromagnetic nature of light and establishes the equation of a wave propagating at the speed of 300,000 km.s^-1.
• 1880s: Heinrich Hertz find conclusive support for the wave nature of light by generating for the first time electromagnetic waves ("radio" waves).

But the experiment dealing with the photoelectric effect poses a problem that the wave theory of light fails to explain. For this experiment, a metal plate (the cathode) is illuminated. Under certain conditions, one measures on another plate (the anode) the electrons ejected from the cathode. A current arises, which is measurable with an ammeter. If light is a wave, the work of Maxwell and Hertz dictates that by increasing the intensity of the electric field (the amplitude of the wave), more electrons would be ejected. A greater current would thus be observed.

That is not what is happening at all. Experimental data reveal that it is the frequency of the incident wave that impacts this current. Below a threshold frequency no electrons are ejected regardless of the intensity of the light.

• 1900s: Based on Max Planck's work on the blackbody and the interaction between heated matter and light emission, Einstein explains the photoelectric effect by considering light as a stream of corpuscles that he calls "photons". Each photon has a quantity of energy (quanta) whose energy (E) is proportional to the frequency (ν) according to Planck's law: E = hν (h is a constant that is 6,626.10^-34  J.s).

The output energy (Φ) is the energy to eject an electron from the cathode. There is therefore a threshold frequency ν₀ below which no photon will be able to eject electrons. Whatever the intensity of the source, for photons of a certain frequency, there is no current. On the other hand, for a photon having a frequency  ν > ν₀  an electron will be ejected and it will even have a kinetic energy corresponding to the difference hν - hν₀. The difference in potential U applied between the cathode and the anode just serves to accelerate (or slow down) these electrons ejected from the cathode.

Einstein was awarded the Nobel Prize in 1921 for his explanation of the photoelectric effect. We must understand the counterintuitive side of this discovery which asserts that light is both a wave and a particle and that certain phenomena are explained only by one or the other of these aspects.