De Broglie Explains the Wave-Particle Duality of Light
Wave-particle duality is a fundamental concept in physics that describes light and matter as exhibiting both wave-like and particle-like properties. Initially, light was considered purely a particle, but experiments in the late 19th and early 20th centuries demonstrated its wavelike nature, notably through Max Planck's work on blackbody radiation. Albert Einstein further contributed to this understanding by explaining the photoelectric effect, which showed that light could behave as particles called photons. Building on these ideas, Louis de Broglie proposed that not only light but also matter has a wave nature, establishing a framework that connected the two phenomena.
De Broglie’s theories suggested that the behavior of electrons in atoms could be described by standing waves, drawing an analogy to Bohr’s quantized energy levels. This insight culminated in his doctoral dissertation and significantly advanced the study of quantum mechanics. Later, Erwin Schrödinger developed a mathematical framework to describe these changing wave patterns, further solidifying the notion of wave-particle duality in matter. The implications of these developments are profound, leading to the foundation of quantum physics and driving numerous technological innovations in the 20th century. This duality remains a cornerstone of modern physics, illustrating the complex nature of light and matter.
De Broglie Explains the Wave-Particle Duality of Light
Date 1923
Louis de Broglie provided a mechanical explanation for the wave-particle duality of light.
Locale University of Paris, France
Key Figures
Louis de Broglie (1892-1987), French prince, historian, and physicistNiels Bohr (1885-1962), Danish physicistErwin Schrödinger (1887-1961), Austrian physicist
Summary of Event
In the early years of the twentieth century, scientists were having difficulty describing the nature of light. For a long time, light had been regarded as acting like a particle. In the late nineteenth century, the wavelike nature of light had been demonstrated. Early in the twentieth century, however, this belief was shifted again by experiments that confirmed the particle nature of light. The wave-particle duality of light was an experimental phenomenon in search of a theory.
At the beginning of the twentieth century, German physicistMax Planck had used the concept of the wave nature of light to explain blackbody radiation (radiation from a theoretical celestial body capable of completely absorbing all radiation falling on it). As a wave, light has a wavelength (the distance between crests) and a corresponding frequency (the number of crests passing a point in a given amount of time). Planck had shown that light of a particular frequency had a definite amount of energy; that is, energy is quantized. This seemed to favor the belief that light was wavelike in nature.
Nevertheless, five years later, in 1905, American physicist Albert Einstein reasoned that light behaved like particles. Einstein used Planck’s theory of quantized light to explain why light striking the surface of certain metals resulted in the ejection of electrons from that metal (the photoelectric effect), but only when this involved certain frequencies of light. He pictured the light striking the metal surface as particles of light, or photons, with sufficient energy to knock off electrons.
The wave-particle nature of light was constantly debated and seemed dependent upon the experiment being performed. For example, the dispersion of white light into its component colors by a prism is a result of the wave nature of light. By contrast, the ability of a stream of photons to eject electrons from a metal surface points to the particle nature of light. Einstein had shown by his relativity theory that light could behave like both waves and particles and that the physical properties of each nature were related. He showed that the momentum of the photon (a particle property) was related to the wavelength of the light (a wave property). Einstein’s results demonstrated that light has wave and particle duality.
Louis de Broglie had been studying Planck’s theories of quantized light and Einstein’s wave-particle concept of light. He wrote several papers calling attention to the dual behavior of light. De Broglie wished to provide a mechanical explanation for the wave-particle duality. Thus he needed to find a mechanical reason for a particle—the photon—to have an energy that was determined by a wave, or rather by the frequency of that wave. While he was thinking about light, de Broglie had the idea that matter (a particle) might have a wave nature also.
At about this time, Niels Bohr had revealed a theory for the electronic structure of atoms. Bohr’s theory was that the electrons in an atom were restricted to particular energy levels and positions called “orbitals.” Only by exact additions of unit amounts of energy could the energy and orbital of an electron be changed.
De Broglie was struck by the analogy of Bohr’s orbital energies to standing waves. As a result, de Broglie discovered an example of wave-particle duality in matter. De Broglie used his explanations of the wave-particle duality of matter in writing his doctoral dissertation in physics, which he presented before the Faculty of Sciences at the University of Paris in 1923. His theory demonstrated that matter, like light, has a wavelike nature.
De Broglie noticed that the momentum of the electron orbitals proposed by Bohr were whole number units of a fundamental quantity, Planck’s constant. He knew that standing waves had unit changes in their momenta also. A standing wave can be thought of as a string, fixed at both ends, that is plucked. The string will oscillate back and forth, yet some points will remain at rest. The number of rest points will increase as the frequency of the vibration increases. De Broglie reasoned that Bohr’s orbitals could therefore be seen as a circular string, a snake swallowing its own tail.
Moreover, de Broglie discovered that the matter waves he had proposed fit Bohr’s electron orbits exactly. He also found that the momenta and wavelengths of his matter waves were related, like those of light. He had succeeded in explaining Bohr’s orbits: Each orbit was a steady wave pattern, and these orbits had determined and fixed sizes so that these distinct “quantized” wave patterns could exist.
When de Broglie somewhat reluctantly submitted his dissertation, the faculty at the University of Paris was unsure of the use of strings to explain Bohr’s orbits and asked Einstein to judge the acceptability of the dissertation. Einstein confirmed that it was sound. The thesis was accepted, and later de Broglie was awarded the Nobel Prize.
Significance
De Broglie’s waves had offered a picture of what was occurring inside an atom. A way to visualize the shifting patterns of the wave was needed when the atom changed energy and produced light. Erwin Schrödinger, an Austrian physicist, found a mathematical equation that explained the changing wave patterns inside an atom. Schrödinger’s equation provides a continuous mathematical description of the wave-particle duality of matter. He viewed the atom as analogous to de Broglie’s vibrating string. The movement of the electron from one orbit to another was a simple change in the frequency of the standing waves of the string. In a musical string, this occurs as the harmony of two wave patterns, the result being the differences in the frequency of the two waves.
The understanding of the wave-particle duality of matter, as modeled by Schrödinger’s equation, was instrumental in the founding of quantum physics. Quantum physics has been responsible for many of the technological advances in the twentieth century. These advances are traceable to de Broglie’s pronouncement of the wave-particle duality of matter.
Bibliography
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