In this blog post, we will look at how Planck’s blackbody radiation theory and Einstein’s photoelectric effect broke through the limitations of classical physics and opened the door to quantum mechanics.
In 1900, Max Planck presented a theory that shook the world, which was dominated by Newtonian physics at the time. Through his theory of blackbody radiation, Planck was the first to explain natural phenomena using quantum theory. Since then, many physicists have developed quantum physics, and the most notable theory among them was the photoelectric effect. The fact that light, which was considered to be only a wave at the time, behaved like a particle and directly affected the momentum of electrons was a major shock. A debate broke out over whether light was a wave or a quantum, and Einstein explained that light had the properties of a quantum through his research on the photoelectric effect. So, what is the photoelectric effect, and how was Einstein able to explain it?
In 1839, before Planck’s theory of blackbody radiation was born, Alexandre Becquerel first discovered the photoelectric effect through conducting solutions exposed to light. However, the physics of the time could not explain this phenomenon. In the late 19th century, the photoelectric effect was re-examined when experimental evidence of light entering certain metal plates causing electrons to be emitted was re-emerged. It was thought that a particle with mass must collide directly with an electron to change its momentum, but many were shocked when light, which was thought to be a massless wave, changed the momentum of an electron. At the time, most people knew that light only had the properties of a wave because it behaved like a particle.
Einstein became interested in this phenomenon and began to study it. At the end of his research, Einstein was able to explain the photoelectric effect, which implied the particle nature of light. Einstein believed that this effect could only occur when light had the properties of a particle, and explained the photoelectric effect by approaching light as a particle. He presented evidence supporting the particle nature of light by explaining the photoelectric effect, for which he was awarded the Nobel Prize in Physics. So, what is the photoelectric effect and how did Einstein explain it?
The photoelectric effect is a phenomenon in which a material such as a metal emits electrons when it absorbs electromagnetic waves with energy above a certain wavelength. Each metal has a unique work function, which is the amount of work required to detach an electron bound within the metal. For example, the work function of sodium is about 2.46 eV, and that of iron is about 4.5 eV. When electromagnetic waves with higher energy than this work function are absorbed by a metal, electrons are immediately emitted. At this time, each metal has a minimum frequency at which electrons can be emitted from the metal, which is called the limit frequency. Let’s take a closer look at how electrons are emitted depending on the strength and frequency of electromagnetic waves.
First, the number of photoelectrons emitted varies depending on the strength of the electromagnetic waves. The stronger the light, the more photoelectrons are emitted. The important point here is that the amount of electrons emitted increases, but the momentum of each electron is unaffected. Second, the energy of the photoelectrons emitted depends on the frequency of the incident light. In the photoelectric effect, the energy of the photoelectrons is expressed as a first-order function of the frequency of the light and the Planck constant.
The stronger the light intensity, the more electrons are emitted, resulting in a large photocurrent. The relationship between the frequency and the electron’s momentum is expressed as a first-order function with Planck’s constant as the slope. Third, at frequencies below the metal’s threshold frequency, no photoelectrons are emitted even if the light is very strong. This is because electrons can only be emitted when electromagnetic waves with high energy that can overcome the work function of metals are incident. Finally, photoelectrons are emitted almost simultaneously with the incidence of light. As if two billiard balls collide, photons, which are light particles, collide with electrons, causing the electrons to be emitted immediately.
Einstein explained these phenomena and won the Nobel Prize in Physics for his work. The reason was that he broke the existing classical physics and presented a turning point in new physics. The photoelectric effect could not be explained by classical physics, and a new approach to physics was needed. Einstein was able to receive the Nobel Prize because he provided this turning point. Then, why couldn’t the photoelectric effect be explained using classical physics?
According to classical physics, as the intensity of light increases, the energy is transferred to the metal plate faster, and the electrons must be released with greater kinetic energy. This is the logic that if you hit the electrons with a stronger force, they will have greater kinetic energy. Also, from the perspective of classical physics, there must be a time lag between the irradiation of light and the emission of photoelectrons, and any light of a certain frequency should emit electrons as long as the light intensity is strong enough. But in reality, it was not the case. The interaction between electrons and light occurred in a way that was not explainable from the perspective of classical physics. No matter how strong the light was, if the frequency was lower than the threshold frequency, no electrons were emitted, but if the frequency was above the threshold frequency, electrons were emitted immediately.
The photoelectric effect from the perspective of classical physics was full of contradictions, which stemmed from the conclusion that light could not have the properties of a particle. With the emergence of not only the photoelectric effect but also de Broglie’s matter wave theory, classical physics gradually lost its power.
With Einstein’s successful explanation of the photoelectric effect, the physics community felt a change in the way they viewed natural phenomena. As various phenomena, such as de Broglie’s discovery of matter waves and the diffraction of electrons, were explained using new approaches rather than classical mechanics, the physics community began to move away from the framework of classical physics. The phenomena that support the particle nature of light and the wave nature of particles came together and became the starting point of quantum mechanics.
However, we still interpret many physical phenomena based on classical physics because it is still efficient in understanding relatively simple everyday physical phenomena. Although classical mechanics cannot explain quantum mechanical phenomena such as the photoelectric effect, it is still meaningful in that it is of great help in understanding simple physical phenomena that can be seen around us.
