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 .
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 .
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.
For all practical purposes the wave theory of light is a certainty. This is the story of a radical change in humans' concept of light. In 1896 most physicists were convinced that light consisted of wave disturbances in a medium, the electromagnetic aether. The energy transported in radiation was thought to propagate spherically outward from its source and to spread out over successively larger volumes in space. Thirty years later, a remarkably different concept of light prevailed. Physicists then took seriously the evidence collected over the preceding two decades showing that the energy of radiation does not spread in space. Under certain conditions, light behaves like a stream of particles. Our subject is more than the story of a shift from one theoretical explanation of radiation to another. This reconsideration of the nature of light was a significant event in our scientific understanding of the world. To resolve the paradox that faced them, physicists rejected the venerable Platonic dictum that the microscopic realm recapitulates the macroscopic; that laws generalized from the behavior of objects in the perceivable world may be applied to the imperceivable one. By 1927 physicists had assigned to all forms of radiation a curious amalgam of wave and particle behavior. Waves spread energy over larger and larger volumes of space; particles do not. Reconciliation of these conflicting properties was possible only through appeal to an ontology that transcended mechanical incompatibility. Nature was declared to be only imperfectly rationalizable in terms of human experience with macroscopic interactions. The programmatic goal formulated in the seventeenth century to reduce all physical phenomena to consistent mechanical representations was here recognized to be unattainable.
Modern attempts to understand light go back to Newton who considered light to be particles, the so called corpuscular theory, and the other school of Huygens, Young and others. Huygens and Young viewpoint emphasised the wave property. This difference of opinions persisted for close to two centuries till Maxwell theory solidly established light as a wave phenomenon associated with Electromagnetism. A serious schism so to speak was introduced into the theory of light with the understanding of "Light gas", the so called Black Body radiation. Einstein's photon hypothesis and Bose's successful derivation of the Planck spectrum put the corpuscular theory back in place, while classical description continues to be used as well. One may then wonder if this being a classical limit may have to be corrected when dealing with quantum phenomena. To most people's surprise, the classical description was in fact a subset of the full quantum description, and the classical states of light could be shown to be subsumed within the fully quantum description without having to take the limit of Planck's constant h going to zero. This was the resolution provided by E. C. G. Sudarshan's Diagonal representation as the most general formalism for dealing with light.
Light - the physicists' problem. The nature of light was the subject \nof such controversy ever since two schools of thought - one led by Newton, \nthe other by Huyghens - had proposed two different viewpoints of the nature \nof light. Newton viewed light as a stream of discrete particles while Huyghens \nmaintained that light was a form of wave-motion. As any O-level student \nknows, experimentation involving wave-properties such as diffraction (the \nfamous Young's slits experiments) eventually proved as correct Huyghens' \nidea of light as waves passing through a continium, which was eventually shown \nto be space itself by the Scot, James Clerk-MaxwelI in his work on electromagnetic \nradiation. By the end of the 19th Century, it seemed that light presented no problem. Light was waves.
FOR twenty years and more we have been hearing continually about the conflict between the corpuscular and the undulatory theories of light, and it is possible that for years to come we may be hearing about a similar contest between the wave-theory and the particle-theory of matter. Furthermore, there are intimations that if an adequate theory either of light or of matter ever is attained, it will involve conceptions of waves which in certain limiting cases approach to conceptions of particles. Already it is established that the appropriate way to attack the typical problems of the atom consists in setting up a wave-equation, and dealing with it in the same manner as one adopts to solve the typical problem of acoustics: how to determine the resonance-frequencies of a piece of elastic matter, such as a taut wire or a drumhead or a column of air in a tube. Therefore it seems opportune to restudy, with care and in detail, the great classical example of a wave-theory highly developed and widely successfulthe great theory of light dimly foreshadowed by Huygens, endowed with its essen tial attributes by Young and Fresnel and Kirchhoff and a host of their coevals, utilized in the design of a multitude of ingenious instruments, perfected by Maxwell and connected with the theory of electricity and magnetism, and serving to this day as the basis for the theory of quanta. So doing, we shall be reminded of many triumphs of the past century of physical research, discoveries which in their time were as exciting as new quantum phenomena in ours; we shall notice certain achievements themselves as recent as those of quanta, and perhaps not less impressive; we shall retrace the reasonings which led to certain conclusions which the quantum-theories, unable to do without them and yet incompetent to derive them, have taken bodily over from their forerunner; we shall reconsider the evidence which in the litigation of a century ago caused the verdict to be rendered in favour of the wave-theory over the particle-theory; and perhaps incidentally we shall be drilling ourselves to test the evidence lately submitted and still to be submitted in the appeal of that case, and in the hearing of that other which impends.
This introductory chapter provides an overview of light. In particle language, a beam of light is a stream of photons—small massless particles with energy and momentum that travel at high speeds. In wave language, a beam of light is a series of waves of changing electric and magnetic field strength that have phase, amplitude, and wavelength, and—like photons—travel at high speeds. To intuitively describe what light does, one has to jump back and forth between these two interpretations. Sometimes, the results of experiments with light are more easily predicted or intuited using the mathematics and metaphors appropriate to waves; and sometimes the results are better explained using the mathematics and metaphors appropriate to particles.