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What is light?

Published May 6, 2021• 0 citations

Abstract

Have you ever tried to define light? If so, your definition is probably related to vision, the ability to see things. The Merriam-Webster English dictionary defines light as “something that makes vision possible”. However, the definition of the “nature of light” is a complex term already discussed by philosophers of Ancient Greece. In the 17th century, the debate on the matter focused on whether light was a particle or a wave, based on the different properties of light that had been gradually discovered over time. Isaac Newton (1642−1727) was a defender of the corpuscular theory of light, which considered light to be formed by particles, i.e., small pieces of matter, like dust or grains of sand. He was the author of Opticks (1706), the front cover of which appears in Fig. 1.1. Christian Huygens (1629–1695), on the other hand, supported the theory that light is a wave. These are two completely different definitions of the same physical concept. A range of experiments were developed to attempt to clarify this dilemma, but it was only until 1900 when Max Planck (1858-1947) introduced the basis of a theory that would go on to revolutionize scientific thinking, marking the beginning of modern physics: quantum theory. Developed during the 20th century thanks to advances made by scientists such as Niels Bohr, Born, Heisenberg, Schrödinger, Pauli, Dirac, Einstein or de Broglie, quantum theory clarified that light is neither a particle nor a wave, but that it has a dual nature: it behaves like a wave as it spreads but like a particle in its interactions with matter. In this book, we will specify where necessary whether light behaves as a particle or as a wave, although in most cases we refer to rays of light, which always represent the direction of propagation of light.

DOI: 10.1117/3.2579764.ch1

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Have you ever tried to define light? If so, your definition is probably related to vision, the ability to see things. The Merriam-Webster English dictionary defines light as “something that makes vision possible”. However, the definition of the “nature of light” is a complex term already discussed by philosophers of Ancient Greece. In the 17th century, the debate on the matter focused on whether light was a particle or a wave, based on the different properties of light that had been gradually discovered over time. Isaac Newton (1642−1727) was a defender of the corpuscular theory of light, which considered light to be formed by particles, i.e., small pieces of matter, like dust or grains of sand. He was the author of Opticks (1706), the front cover of which appears in Fig. 1.1. Christian Huygens (1629–1695), on the other hand, supported the theory that light is a wave. These are two completely different definitions of the same physical concept. A range of experiments were developed to attempt to clarify this dilemma, but it was only until 1900 when Max Planck (1858-1947) introduced the basis of a theory that would go on to revolutionize scientific thinking, marking the beginning of modern physics: quantum theory. Developed during the 20th century thanks to advances made by scientists such as Niels Bohr, Born, Heisenberg, Schrödinger, Pauli, Dirac, Einstein or de Broglie, quantum theory clarified that light is neither a particle nor a wave, but that it has a dual nature: it behaves like a wave as it spreads but like a particle in its interactions with matter. In this book, we will specify where necessary whether light behaves as a particle or as a wave, although in most cases we refer to rays of light, which always represent the direction of propagation of light.

The phenomena of light is one of the phenomena that mankind started to study most early. According to some researches, first lamp was made 70 000 years ago (it was made of some materials that could burn and on that way, made light). It is hard to imagine that mankind could not explain light until the year 1905. From 17th century until the 20th century light was scientific studied because some question related with light could not be explained. For over 350 years scientists have been trying to figure out about strange nature of light that makes it to behave sometimes as wave and sometimes as particle. Today is confirmed The dual theory of light – wave and corpuscular theory and depending on which phenomena we study, light can manifest as a wave or as a particle.

Scientific inquiry took millennia to ascertain the true nature of light, and it is not possible here to attempt to recapitulate all of the philosophizing, hypotheses, arguments, and experimentation involved in that great debate. The main point of controversy was whether light should be considered as a particle or as a wave. Empedocles of Acragas (492–432 b.c.e .) was of the opinion that light is a “streaming substance” emitted by luminous bodies, and that light travels at a finite speed ( Sambursky, 1958 ). In the seventeenth century, Isaac Newton postulated that light is composed of particles and that color is connected with the size of those particles; however, his contemporary Christiaan Huygens argued that light is a wave that transfers energy but not substance ( Einstein & Infeld, 1966 , p. 105ff). It turns out that light is indeed a wave propagating via electric and magnetic fields, but in addition to exhibiting properties of waves (such as interference) it also exhibits properties of particles (such as momentum and localization). When treating light as particles, the individual quanta are called photons .

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Focusing on the unresolved debate between Newton and Huygens from 300 years ago, The Nature of Light: What is a Photon? discusses the reality behind enigmatic photons. It explores the fundamental issues pertaining to light that still exist today. Gathering contributions from globally recognized specialists in electrodynamics and quantum optics, the book begins by clearly presenting the mainstream view of the nature of light and photons. It then provides a new and challenging scientific epistemology that explains how to overcome the prevailing paradoxes and confusions arising from the accepted definition of a photon as a monochromatic Fourier mode of the vacuum. The book concludes with an array of experiments that demonstrate the innovative thinking needed to examine the wave-particle duality of photons. Looking at photons from both mainstream and out-of-box viewpoints, this volume is sure to inspire the next generation of quantum optics scientists and engineers to go beyond the Copenhagen interpretation and formulate new conceptual ideas about light-matter interactions and substantiate them through inventive applications.

The behavior of light and its interaction with matter is certainly one of the most interesting phenomena of physics. It has puzzled thinkers and scientists in ancient Greece (Archimedes) and through the Middle Ages, at least since the times of Robert Grosseteste (1168–1235), right up to the theories of Galileo Galilei and Isaac Newton. The classical electromagnetic theory of light (see Tab. 13.1) was beautifully established in the second half of the nineteenth century with the Maxwell equations. Quantum optics is nothing other than the quantum theory of light. It finds its roots in Planck's discovery (1900) that the assumption of energy quantization for a harmonic oscillator allowed us to solve the black-body problem (see Subsec. 1.5.1). The successive early steps that led to the quantization of the electromagnetic field include Einstein's explanation of the photoelectric effect with the introduction of the quantum of light (see Subsec. 1.2.1) and the interpretation of atomic spectra (see Subsec. 1.5.4). Quantum optics is usually not covered by conventional textbooks on quantum mechanics. We include its discussion here for two main reasons: first, because it is a fundamental and advanced application of the basic framework of quantum mechanics, and, second, because in the last 30 years quantum optics has played (and is still playing) a major role in the advancement and understanding of quantum mechanics.