This blog post explores how Planck’s blackbody radiation theory and Einstein’s photoelectric effect transcended the limitations of classical physics and opened the door to quantum mechanics.
In 1900, Max Planck presented a theory that shook the world dominated by Newtonian physics. Through his blackbody radiation theory, Planck offered the first quantum explanation for a natural phenomenon. Subsequently, numerous physicists developed quantum physics, with the photoelectric effect becoming one of its most celebrated theories. The revelation that light, previously considered solely a wave, behaved like particles and directly influenced the momentum of electrons was profoundly shocking. A debate erupted over whether light was a wave or a particle, and Einstein, through his own research on the photoelectric effect, explained that light possesses particle-like properties. So, what exactly is the photoelectric effect, and how was Einstein able to explain it?
In 1839, before Planck’s blackbody radiation theory emerged, Alexandre Becquerel first discovered the photoelectric effect through a conductive solution exposed to light. However, the physics of the time could not explain this phenomenon. In the late 19th century, the photoelectric effect was revisited when experimental evidence reappeared showing that light incident on specific metal plates caused the emission of electrons. It was believed that changing an electron’s momentum required direct collision with a particle possessing mass. Yet when light—thought to be a massless wave—altered the electron’s momentum, many were stunned. Most people at the time understood light to possess only wave properties, so the fact that light behaved like a particle was astonishing.
Einstein became intrigued by this phenomenon and began his research. Ultimately, Einstein explained the photoelectric effect, which implied the particle nature of light. Einstein believed this effect could only occur if light possessed particle-like properties. He approached light as a particle to explain the photoelectric effect. By explaining the photoelectric effect, he provided evidence supporting the particle nature of light, 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 the phenomenon where a material, such as a metal, emits electrons when it absorbs electromagnetic waves with energy above a specific wavelength. Each metal possesses a unique property called the work function, which represents the amount of work required to free an electron bound within the metal. For example, sodium has a work function of approximately 2.46 eV, while iron has one of about 4.5 eV. When electromagnetic waves with energy higher than this work function are absorbed by the metal, electrons are immediately emitted. At this point, each metal has a minimum frequency at which electrons can be emitted, known as the threshold frequency. So, let’s examine in detail how electrons are emitted depending on the intensity and frequency of the electromagnetic waves.
First, the number of photoelectrons emitted varies with the intensity of the electromagnetic wave. The stronger the light, the more photoelectrons are emitted. A key point here is that while the total number of emitted electrons increases, the momentum of each individual electron remains unaffected. Second, the energy of the emitted photoelectrons changes depending on the frequency of the incident light. In the photoelectric effect, the energy of the photoelectrons emitted by the incident light is expressed as a linear function with the light’s frequency and Planck’s constant as the slope.
The stronger the light intensity, the more electrons are emitted, generating a larger photoelectric current. The relationship between frequency and electron momentum is expressed as a linear function with Planck’s constant as the slope. Third, at frequencies below the metal’s threshold frequency, no photoelectrons are emitted regardless of how intense the light is. This is because electrons can only be emitted when electromagnetic waves with sufficient energy to overcome the metal’s work function are incident. Finally, photoelectrons are emitted almost instantaneously upon light incidence. Much like two billiard balls colliding, the light particle (photon) collides with the electron, causing the electron to be emitted instantly.
Einstein explained these phenomena and was awarded the Nobel Prize in Physics for this achievement. The reason was that he broke with existing classical physics and presented a turning point for a new physics. Classical physics could not explain the photoelectric effect, necessitating a new approach to physics. Einstein received the Nobel Prize because he provided this turning point. So why couldn’t classical physics explain the photoelectric effect?
According to classical physics, increasing light intensity should deliver energy to the metal plate more rapidly, causing electrons to be emitted with greater kinetic energy. This follows the logic that striking an electron with a stronger force should impart greater kinetic energy to it. Furthermore, from a classical physics perspective, there should be a time delay between light irradiation and photoelectron emission, and electrons should be emitted for light of any frequency as long as its intensity is sufficiently strong. But this was not the case in reality. The interaction between electrons and light occurred in a way that was incomprehensible from the viewpoint of classical physics. No matter how intense the light, if its frequency was below the threshold frequency, no electron was emitted; but as soon as the frequency exceeded the threshold, an electron was emitted immediately.
The photoelectric effect, viewed through classical physics, was riddled with contradictions, stemming from the assumption that light could not possess particle-like properties. Not only the photoelectric effect, but also the emergence of de Broglie’s matter-wave theory, gradually eroded the authority of classical physics.
As Einstein successfully explained the photoelectric effect, the physics community began to sense a shift in how natural phenomena were perceived. With phenomena like de Broglie’s discovery of matter waves and electron diffraction being explained successively by new approaches beyond classical mechanics, the physics community showed a movement to break free from the framework of classical physics. As phenomena supporting the particle nature of light and the wave nature of particles accumulated, this became the starting point for quantum mechanics.
However, we still interpret many physical phenomena based on classical physics. This is because classical physics remains efficient for understanding relatively simple, everyday physical phenomena. Although classical mechanics cannot explain quantum mechanical phenomena like the photoelectric effect, it still holds significance in that it greatly aids our understanding of the simple physical phenomena observable around us.
