How Planck’s black-body radiation theory and Einstein’s photoelectric effect shook up classical physics!

In 1900, Max Planck marked the beginning of quantum physics with his black-body radiation theory in an era dominated by Newtonian physics. Einstein broke the limits of classical physics by proving the particle nature of light through the photoelectric effect. This marked an important turning point in the birth and development of quantum mechanics.

 

In 1900, Max Planck proposed a theory that would shake up the world dominated by Newtonian physics at the time. With his theory of blackbody radiation, Planck gave the first quantum explanation of natural phenomena. Numerous physicists have since developed quantum physics, the most notable of which was the photoelectric effect. The discovery that light, which was only considered a wave at the time, could behave like a particle and directly affect the momentum of electrons was a huge shock. The debate raged over whether light was a wave or a quantum, and Einstein used his work on the photoelectric effect to explain that light had quantum properties. So, what is the photoelectric effect, and how did Einstein explain it?
In 1839, before Planck’s theory of blackbody radiation, Alexandre Becquerel first discovered the photoelectric effect in a conducting solution exposed to light, but the physics of the time could not explain the phenomenon. In the late 19th century, the photoelectric effect was rediscovered when experimental evidence reappeared that light incident on certain metal plates caused electrons to be emitted. It was thought that a particle with mass must directly collide 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 thought that light only had wave-like properties because it behaved like a particle.
Einstein was intrigued by this phenomenon and began to study it. His research led him to describe the photoelectric effect, which implied the particle nature of light. Einstein believed that this effect could only occur if light had particle properties, and he explained the photoelectric effect by approaching light as a particle. By explaining the photoelectric effect, he provided evidence to support the particle nature of light and was awarded the Nobel Prize in Physics for his work. 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 property called its own work function, which refers to the amount of work required to release the electrons trapped within the metal. For example, sodium has a work function of about 2.46 eV and iron has a work function of about 4.5 eV. When an electromagnetic wave with an energy higher than this work function is absorbed by a metal, the electrons are immediately released. 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 the electromagnetic wave.
First, the strength of the electromagnetic wave determines the number of photoelectrons emitted. The stronger the light, the more photoelectrons are emitted. It’s important to note that the amount of electrons emitted only increases, but the momentum of each electron is not affected. Second, the energy of the emitted photoelectrons changes depending on the frequency of the incident light. In the photoelectric effect, the energy of a photoelectron from incident light is a first-order function of the frequency of the light and Planck’s constant as a slope.
The stronger the light intensity, the more electrons are emitted, resulting in a large photocurrent, and the relationship between the frequency and the momentum of the electrons is a first-order function of the slope of Planck’s constant. Third, at frequencies lower than the metal’s limit frequency, no matter how intense the light, no photoelectrons are emitted. This is because electrons can only be emitted when an electromagnetic wave with a high enough energy to overcome the metal’s work function is applied. Finally, the photoelectrons are emitted almost simultaneously with the incident light. Like two billiard balls colliding, a photon, a particle of light, collides with an electron and the electron is immediately released.
Einstein’s explanation of these phenomena won him the Nobel Prize in Physics. This is because he broke with classical physics and marked a turning point in physics. Classical physics couldn’t explain the photoelectric effect, and a new approach to physics was needed. Einstein was awarded the Nobel Prize because he provided this turning point. So, why couldn’t classical physics explain the photoelectric effect?
According to classical physics, as the intensity of the light increases, energy is transferred to the metal plate faster, and the electrons should be released with greater kinetic energy. The logic is that if you hit an electron with more force, it will have more kinetic energy. Also, from a classical physics perspective, there should be a time lag between the irradiation of light and the emission of photoelectrons, and light of any frequency should emit electrons as long as the intensity of the light is strong enough. But in practice, this is not the case. The interaction between electrons and light was not consistent with classical physics. No matter how strong the light, if its frequency was below the limit frequency, no electrons were emitted, but if it was above the limit frequency, electrons were emitted immediately.
The photoelectric effect from the point of view of classical physics was full of contradictions, stemming from the conclusion that light could not have the properties of a particle. With the advent of not only the photoelectric effect, but also De Broglie’s theory of matter waves, classical physics gradually lost its power.
With Einstein’s successful explanation of the photoelectric effect, the physics community realized a change in the way they viewed natural phenomena. De Broglie’s discovery of matter waves, the diffraction of electrons, and other phenomena were explained by new approaches other than classical mechanics, and the physics community began to move away from the framework of classical physics. The convergence of phenomena that supported the particle nature of light and the wave nature of particles led to the development of quantum mechanics.
However, we still interpret many physical phenomena based on classical physics. This is because classical physics is still effective in understanding relatively simple, everyday physical phenomena. Although classical mechanics cannot explain quantum phenomena such as the photoelectric effect, it is still very helpful in understanding the simple physical phenomena we see around us.