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Institute of Physics eBooks

Classical nature of light

Muhammad Sabieh Anwar, Faizan-e-Ilahi, Syed Bilal Hyder, Muhammad Waseem

Published Aug 1, 2024• 0 citations
Publisher

Abstract

The primary subject of this book is photons, the tiniest units of light. The behaviour of photons is described by quantum physics, but to see this fully, we first discuss the classical properties. The chapter also describes experiments that reveal interference and polarization of light and is a useful prelude to what follows.
DOI
10.1088/978-0-7503-6315-0ch2

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Humans perceive the world at a macroscopic level where the laws of nature can be explained using classical physics. However, at the sub-atomic level, systems exhibit phenomena that are not explainable using classical physics alone. To model the behaviour of these tiny systems, the pioneers developed a framework known as quantum mechanics. Since the main focus of this book is photons, we must understand basics of quantum mechanics before studying the behaviour of photonic systems. This mechanics has a special mathematical formalism which is the subject of this chapter.
The previous chapter surveyed part of the rich variety of physical phenomena that can be understood using the ideas of quantum physics. Now that we are beginning the task of looking more deeply into the subject we shall find it useful to concentrate on examples that are comparatively simple to understand but which still illustrate the fundamental principles and highlight the basic conceptual problems. Some years ago most writers discussing such topics would probably have turned to the example of a ‘particle’ passing through a two-slit apparatus (as in Figure 1.2), whose wave properties are revealed in the interference pattern. Much of the discussion would have been in terms of wave–particle duality and the problems involved in position and momentum measurements, as in the discussion of the uncertainty principle in the last chapter. However, there are essentially an infinite number of places where the particle can be and an infinite number of possible values of its momentum, and this complicates the discussion considerably. We can illustrate all the points of principle we want to discuss by considering situations where a measurement has only a small number of possible outcomes. One such quantity relating to the physics of light beams and photons is known as polarisation . In the next section we discuss it in the context of the classical wave theory of light, and the rest of the chapter extends the concept to situations where the photon nature of light is important.
The previous chapter surveyed part of the rich variety of physical phenomena that can be understood using the ideas of quantum physics. Now that we are beginning the task of looking more deeply into the subject we shall find it useful to concentrate on examples that are comparatively simple to understand but which still illustrate the fundamental principles and highlight the basic conceptual problems. Some years ago most writers discussing such topics would probably have turned to the example of a ‘particle’ passing through a two-slit apparatus (as in Figure 1.2), whose wave properties are revealed in the interference pattern. Much of the discussion would have been in terms of wave–particle duality and the problems involved in position and momentum measurements, as in the discussion of the uncertainty principle in the last chapter. However, there are essentially an infinite number of places where the particle can be and an infinite number of possible values of its momentum, and this complicates the discussion considerably. We can illustrate all the points of principle we want to discuss by considering situations where a measurement has only a small number of possible outcomes. One such quantity relating to the physics of light beams and photons is known as polarisation . In the next section we discuss it in the context of the classical wave theory of light, and the rest of the chapter extends the concept to situations where the photon nature of light is important.
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.
It is evident from the previous chapter that certain quantities that appear continuous at the macroscopic scale does actually exist in a discrete form at the microscopic scale. So, quantization of these entities makes sense and reveals a plethora of information at such length scales. In this book, our key concern lies with the properties of light, and rightfully so, we will explore different states of the optical radiation field. We will learn that light exists in many states that makes it so unique. Experimental evidence shows that light behaves as a particle at the microscopic scale (subject to specific conditions), although it behaves like a wave at the macroscopic scale. In this chapter, we will dwell deep and discuss many characteristics related to the particle nature of light. Specifically, we will consider two important states of light, the Fock and coherent states. These states can be regarded as the most fundamental states of light that enable complex interpretation of quantum features in the radiation field.
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