The photoelectric effect

The photoelectric effect was one of the levers that propelled the merge of quantum mechanics, being intrinsically linked to Max Planck's quanta theory (already discussed in past articles). In the 19th century, Heinrich Hertz developed a particularly curious experiment, which consisted of two metal plates, separated by a small distance, subjected to vacuum inside a glass tube. The plates were then connected to each other by a conductive wire with an ammeter (device that measures the electrical current intensity, unit given in the IS by Coulomb/second), in order to create an electrical circuit. Afterwards, one of the plates receives light, while the other does not. The plate that received radiation (light) ejects electrons from its surface, which reach the other plate, generating an electric current. Scientists realized that different metals needed different amounts of energy in order for electrons to be expelled from the metal surface. What intrigued the scientific community were certain observations on the effects that light caused, differing from predictions pointed by already consolidated theories, such as classic electromagnetism (theory that explains the behavior of electromagnetic waves, that is, light), formalized by James Clark Maxwell. When increasing the amplitude (intensity) of the emitted light, it was noticed that the electrons did not come out more energized (with greater speed), as would be expected. Exemplifying: the incidence of both faint/weak and strong light emitted electrons with equal kinetic energy. The effect that the luminous intensity had was to remove a greater amount of electrons from the metallic surface, thus increasing the electric current generated, given the greater amount of particles (electrical charges). However, the amplitude had no impact on the energy of the electrons emitted. Physicists also had predicted that increasing the frequency of photons would cause the electric current to increase (what was seen to be the role of light amplitude), but, surprisingly, experiments showed that the increased frequency generated electrons emitted with greater energy (faster). Yet another stalemate challenged the perception of scholars. The emission of electrons should, after the incidence of light on the metal plate, take a few instants to occur, as the wave behavior would prescribe. However, it was observed that the electron's expulsion was instantaneous, which pointed to a corpuscular/particle behavior. It is important to emphasize that not every light frequency is able to remove the electrons; lower frequencies (reddish colors in the visible spectrum), in general, are insufficient for the removal, whereas higher frequencies (colors closer to violet in the visible spectrum) easily strip off electrons. The explanation for the erroneous predictions came later, given by Albert Einstein in 1905, in the so called “miraculous year” (year of the publication of Einstein's famous papers). Max Planck had previously explained that energy is not transported continuously, but in “energy packages” (so-called quanta) and is therefore quantized (cannot assume any arbitrary value). Einstein used Planck's idea and proposed that light is composed of particles, called photons, which carry a discrete (quanta) portion of energy. Einstein's explanation for the photoelectric effect, over time, gained acceptance among the scientific community, guaranteeing Einstein, in 1922, the Nobel Prize in physics. In summary, the photoelectric effect occurs when photons (of certain allowed frequencies) are absorbed by electrons on the metal surface, supplying them with energy, in order to pull them out of the plate. The photon energy is given by Planck's law (equation 1), and the photon energy can also be expressed by the energy conservation law (equation 2), as the sum of the kinetic energy (of motion) and the work done to expel the electron. Conservation is due to the fact that the input energy (of the photon) is equal to the energy used to do work and give movement to the electrons (called photoelectrons when expelled from the plate). From the implications of the photoelectric effect, there is the indication that light has a corpuscular behavior, contradictings previous experiments, such as Young's double slit, which had shown that light displayed a wave-like behavior. The simultaneity of both behaviors affirms the wave-particle duality, one of the fundaments of quantum mechanics. The proof that light is composed of particles brought to reality the existence of bodies without mass. Furthermore, with the implementation of Planck's theory, Einstein helped to kick-start what would flourish in the most curious and mysterious theory ever formulated by mankind, the quantum theory.