Wave or Particle?
The earliest comprehensive theories on the nature of light were advanced around the turn of the 17th century. In 1690, Christiaan Huygens put forward a wave theory of light, demonstrating how light waves might superimpose (interfere) to form a wavefront, similar to water waves (Fig 2-2), and travel in a straight line (in accordance with the ray approximation of geometric optics). As such, Huygens’s wave theory covered little of what we would call physical optics today and was soon overshadowed by Isaac Newton’s particle viewpoint. Newton suggested that light was made of particles—he called them “corpuscules.” The particle theory of light took precedence and was accepted essentially unchallenged for over a century.
In the early 19th century, Thomas Young’s double-slit experiment appeared to prove the wave-like behavior of light, via the observation of bright and dark bands (fringes) upon illumination of a barrier with 2 narrow openings, very similar to the interference pattern that results from the superposition of water waves. Later that century, James Clerk Maxwell formulated light entirely as a wave; that is, as the propagation of electromagnetic waves (Fig 2-3A) according to his Maxwell equations. Thus, for many years after Newton, as interference, diffraction, and other phenomena were readily explained by waves, the wave model became the dominant theory of the nature of light.
However, it is now clear that light also exhibits properties of particles, as seen in experiments with instruments that are sensitive enough to detect very weak light. An example of such an instrument is a photomultiplier, which produces “clicks” when light is shone on it. According to the wave theory, these clicks should become less loud as the intensity of the impinging light is decreased. In reality, these clicks remain equally loud, but decrease in frequency. Thus, light can be thought of as something like raindrops, with each little “drop” being called a photon.
Figure 2-2 A wavefront is an imaginary construct that can best be understood in the example of water waves. Note that wavefronts are perpendicular to the ray direction.
(Photograph by Sang Pak, modified by Kristina Irsch, PhD.)
Figure 2-3 Electromagnetic wave theory and quantum theory. A, According to electromagnetic wave theory, light may be described as a transverse electromagnetic wave, whose oscillating electric (E) and magnetic (H) fields are both perpendicular to each other as well as to the direction that the light travels. The wavelength (λ) and frequency (ν) of light are related to each other via the “speed of light” (c = νλ ≈ 3 × 108 m/s in vacuum). B, According to quantum theory, electrons in atoms (or molecules) exist in nonradiating states, with each state being associated with a specific energy level. In a process called stimulated absorption, an electron moves between its lowest energy (ground) state (E0) and an excited state (E1) after picking up (absorbing) a photon or quantum of energy (E = E1 – E0 = hν). Similarly, a photon is given off (emitted) when an electron moves from a higher energy state to a lower one (spontaneous emission). h is the Planck constant.
(Illustration part A by Kristina Irsch, PhD. Part B Reproduced with permission from Steinert RF, Puliafito CA. The Nd:YAG Laser in Ophthalmology: Principles and Clinical Applications of Photodisruption. Philadelphia: Saunders; 1985. Redrawn by C. H. Wooley and modified by Kristina Irsch, PhD.)
In the early 20th century, Albert Einstein postulated light as the sum of individual packets (quanta) of energy, and thus was born the word photon and the quantum theory of light (Fig 2-3B). This influenced the formation of the concept of a wave–particle duality of light, which was not a very satisfactory theory, but rather represented a state of confusion, as to exactly which model to use in each circumstance. Maxwell’s theory of electromagnetism (which fails to explain the observations with very dim light falling on photomultipliers) had to be modified to conform with the concepts of quantum mechanics (essentially a description of the motion of electrons in matter).
A new theory—quantum electrodynamics (QED)—the quantum theory of the interaction of light and matter, developed by a number of physicists in 1929 and clarified by Richard Feynman and 2 other physicists, Julian Schwinger and Sin-Itiro Tomonaga, in 1948, unites Maxwell’s equations of electricity and magnetism with quantum mechanics. It thereby resolves the wave–particle confusion by stating that light is made of particles, not in the ordinary sense to be able to predict exactly what will happen in any given experiment, but rather it leaves us with being able to calculate only probabilities of an event.
Thus, contrary to what is taught in many ophthalmology and ophthalmic optics textbooks, there is one theory that explains all the properties of light we know: QED. Note that for most purposes, in practice it suffices to consider the wave-like behavior of light (see Fig 2-3A) for phenomena at the macroscopic level, and the simple quantum-view of the light (see Fig 2-3B) at the microscopic level, as will be illustrated.
Excerpted from BCSC 2020-2021 series : Section 3 - Clinical Optics. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.