Chapter 3 reviews the traditional historical, schoolbook notions of thermal radiation quanta found in the work of Planck, the traveling and absorbed quanta of Einstein, the spectroscopic evidence associated with the atom transitions of Bohr, and the scattering evidence of Compton – experiments that resulted in seemingly conicting views of photons as particles and as waves. It comments on revisionist views that have made these four experimental works no longer the definitive evidences for photons that they once were regarded.
Chapter 3 reviews the traditional historical, schoolbook notions of thermal radiation quanta found in the work of Planck, the traveling and absorbed quanta of Einstein, the spectroscopic evidence associated with the atom transitions of Bohr, and the scattering evidence of Compton – experiments that resulted in seemingly conicting views of photons as particles and as waves. It comments on revisionist views that have made these four experimental works no longer the definitive evidences for photons that they once were regarded.
This book describes the changing views of the physics community toward photons, and how photons are viewed today in several contexts. The first portion, a ninechapter Memoir with few equations and many definitions, explains the changing view of physicists toward radiation and its wave-particle photons, written for those with interest but possibly without technical background. It gives operational definitions that have been used for photons and their association with quantum-state manipulations that include Quantum Information, astronomical sources and crowds of photons, the boxed fields of cavity Quantum Electrodynamics It defines, qualitatively, the historical photons of Planck, Einstein, Compton, and Bohr, the later photons of Dirac, Feynman, and Glauber, and the photon constituents of the Standard Model of Particle Physics. It points to contemporary photons as causers of change to atoms, as carriers of messages, and as subject to controllable creation and alteration. A second portion, of three tutorial appendices, explains the mathematical background of quantum theory and radiation needed by those whose profession involves photonics and who therefore want more detailed understanding of the Memoir portion: quantum theory and the Schrodinger equation for quantum-state manipulation; Maxwell equations for electromagnetism with wave modes that become photons through a quantization postulate, possibly exhibiting quantum entanglement; and the coupling of atoms and fields to create quasiparticles that are seen as slow and stored light pulses. As with other Memoirs, the present book has idiosyncrasies of the author. Most notably, on the opening page of each chapter, and at the end of the book, is a cartoon drawn by the author, as a grad student, that reflects the changing views of a PhD aspirant toward the grad school experience as he progressed through the graduate school of MIT in the 1950s.
This chapter examines the historical development of quantum mechanics, focusing on key experiments and theories that challenged classical physics. It begins with blackbody radiation, highlighting the discrepancy between classical predictions and experimental observations, leading to Planck's quantization hypothesis, which posits that energy is emitted in discrete packets (quanta). The chapter covers the photoelectric effect and Compton scattering, demonstrating wave–particle duality and validating quantum theory. It also discusses atomic models, culminating in the Bohr model and the energy-level diagram of the hydrogen atom, which explains atomic phenomena such as line spectra and discrete energy transitions. The chapter concludes by introducing the quantum model, which supersedes the Bohr model by incorporating wave–particle duality and the uncertainty principle, highlighting the necessity of quantum mechanics for accurate atomic-scale descriptions.
This chapter starts with the historical development of quantum mechanics and explanations of key concepts behind fundamental discoveries such as Planck's hypothesis, the photoelectric effect, Bohr's atomic model, Compton scattering, the de Broglie hypothesis, the Pauli exclusion principle, the Schrödinger equation, Heisenberg quantum mechanics and the uncertainty principle, the Born interpretation of wave function, the Davisson and Germer experiment, and Dirac relativistic quantum mechanics, which made important contributions to the foundation of modern quantum mechanics. Concepts like the particle properties of radiation, the wave properties of matter, quantized energy and momentum, and the idea that one can no longer know exactly where a single particle like an electron is at any instant of time are brought out clearly through this chapter. A list of important quantum mechanical parameters along with solved examples and unsolved exercises on the key concepts are included towards the end.
This chapter begins with a survey of early experimental results — the Planck spectrum, the photoelectric effect, and Compton scattering — that are usually presented as evidence for the existence of photons. The photon concept is indeed sufficient to explain these results, but it is not necessary. There are semi-classical models that explain the same data without introducing photons. The crucial experiment, in which light consisting of a single photon reflects from a beam splitter, depends on the essential indivisibility of photons, and it excludes all semi-classical descriptions of light. This discussion is followed by a preview of modern methods of production and detection of individual photons, e.g., spontaneous down conversion and Silicon avalanche-photodiode counters, together with an introduction to the quantum theory of light based on the correspondence principle.
Einstein never deviated from his belief in the reality of light quanta. In 1909, he published a remarkable paper in which he showed that fluctuations in the intensity of black-body radiation consisted of the statistical sum of the wave and particle properties of light. Following the 1911 Solvay conference, most of the participants were converted to the existence of light quanta. Bohr first applied the concepts of quanta to the structure of atoms. The experiment which finally convinced everyone of the light quantum hypothesis was Compton's demonstration of the energy change of X-rays in their scattering by electrons. This resulted in the concept of the wave-particle duality of light, one of the major foundation stones of quantum mechanics.