For centuries, the nature of light has been a source of much wonder and study. The fascinating nature of light lends the spectacular vividness to the natural world that our senses perceive. The colour of the sky embellished with bluish and reddish hues, the glittering mosaic of rainbow layers bejeweled with iridescence, and the glamoring surface of lakes reflecting flashes of sapphire wonder are all remarkable manifestations of the properties of light.
While the study of the nature of light might not appear to have any inherent materialistic value at first, such a pursuit has nonetheless underpinned major technological advances that we depend upon in our everyday lives, things of the calibre of lasers, computers, TVs, and medical imaging devices. Indeed, cameras record pictures using light and images, videos, and music files on DVDs are all read using light.
The understanding of light itself has been mired in deep historical developments, ones that have spurred unimaginable realms of knowledge, enabled major insights, and inspired new lines of thought. It would be safe to say that nothing in physics makes sense except in the light of light.
The study of light begins with the ancient Greeks. According to Aristotle, light is not a substance. It is a state of transparency of air or water that is not produced by movement but rather by an instantaneous change in state that causes change of colour in transparent medium and a consequent production of light. The action of fire is what actualizes transparency and accords colour to medium. At the time, this theory garnered a wider acceptance than Democritus’s theory that light consists of small invisible particles emitted from a source and that move at a finite speed and interact with each other.
The Greek philosopher Ptolemy (100 – 170 AD) described light as a form of energy. Using Euclid’s geometric methods, he hypothesized that light proceeds from the eye as a continuous visual cone that can be abstracted by discrete lines. The continuous nature of Ptolemy’s cone was able to account for the fact that objects at a distance do not come in and out of existence and that objects could still be seen without having to move the eyeballs accordingly.
In the 1000s AD however, the Arab scholar Alhazen was able to decisively dispel Ptolemy’s concept of extramission by a series of experiments published in his seven volume De Aspectus, where he argued the light originates not from the eye but from an object as that of the sun and then is reflected from the object of interest and into the eye. More importantly, Alhazen experimented with the camera obscura (pinhole camera), where light shone into a small hole would form an inverted image. Such experiments showed that light traveled in straight rays.
As the 17th century unfolded, two ideas about the nature of light would gain ground and persist for 200 years. The Dutch physicist, Christiaan Huygens, in his 1678 The Treatise on Light treated light as a stream of spherical waves emitted from a luminous source and that vibrated parallel to the direction of light travel. Each point on the wave front was a secondary disturbance that propagated outwards with the same frequency and phase in all directions. Since waves have to travel in a medium, Huygens propounded the hypothesis that light propagated in some sort of mysterious luminiferous ether that permeates the universe and possesses the properties of an elastic solid. In The Treatise on Light, Huygens says:
“We know, that by means of air, which is an invisible and impalpable body, sound spreads through the whole of space surrounding its source by a motion which advances gradually from one air particle to the next, and since the propagation of this motion takes place with equal speed in all directions, spherical surfaces must be formed that spread out further and further, finally to reach our ears. Now it is beyond doubt that light also reaches us from luminous bodies by means of some motion which is imparted to the intermediate matter, for we have already seen that this could not have happened by means of the translation of a body that might have reached us from there. If now, as we shall soon investigate, light needs time for its path, it follows that this motion imparted to matter must be gradual, and that, like sound, it must spread in spherical surfaces or waves; I call them waves because of their similarity to those which we see being formed in the water when a stone is thrown into it, and because they enable us to observe a like gradual spreading-out in circles, although they are due to a different cause and only form in a plane surface.”
Huygens wave theory of light was able to account for the fact that light rays can cross one another without impeding one another. Huygens reasoned that light appears to travel in a straight line when shone through an aperture because the secondary propagations produced through the aperture reinforce each other into straight travel. Indeed, Huygens’ model was predicated on the notion that secondary wavelets combine and their envelopes build on to form the wavefronts. It was, thus, able to explain reflection and refraction at surfaces of media. Huygens reasoned that “if the particles of transparent bodies have a recoil a little less prompt than that of the ethereal particles, which nothing hinders us from supposing, it will again follow that the progression of the waves of light will be slower in the interior of such bodies than it is outside in the ethereal matter.”
However, one problem with Huygens’ theory was that it failed to adequately explain rectilinear propagation. Huygens had regarded light as highly akin to sound, similarly consisting of waves. Light waves would, accordingly, be expected to curl around corners. This presented a major hurdle for Huygens’ wave theory of light, which assumed spherical propagation of waves from source. Huygens had attempted to explain that by applying geometrical properties to the behaviour of light waves. But, his model was still largely inadequate in that respect and many of his assumptions were unfounded.
Huygens’ model was not shared by his contemporary of the time and rival, Sir Isaac Newton. Newton had proposed his corpuscular theory of light; that light consists of a stream of weightless particles moving at high speeds and subject to the forces of inertia and gravity. He reasoned that the corpuscular nature of light is evident in the fact that light does not bend when hitting obstacles. Newton, accordingly, explained reflection by modeling light as corpuscles that bounce off a hard surface. He explained that denser medium would have a stronger gravitational pull to account for refraction. Newton quickly began to discount Huygens’ theory and recognised some of the difficulties it had posed, in particular, the notion that light waves would spread outward and therefore would be expected to be seen around corners and the notion that light traveled in straight lines, unlike what waves in a hypothetical etherial medium would do. Although Huygens’ theory was consistent with some of the early experiments, it had largely paled in comparison due to Isaac Newton’s preeminent and highly revered stature at the time. In order to detract from his rivals, Newton’s experiments fell short of incorporating contemporary hypotheses and assumptions. Indeed, his Opticks starts with the statement: “My Design in this Book is not to explain the Properties of Light by Hypotheses, but to propose and prove them by Reason and Experiments“.
Nevertheless, in the ensuing years, many of the properties of light that Newton would discover would lend credence to the wave nature of light. Even much earlier, in 1666, Newton had discovered that white light could be broken up into constituent colours that were not an artefact of instrumentation as previously thought. In his darkened chamber, he carried out his Experimentum crucis. He juxtaposed two prisms, between which he positioned two boards, each pierced by a small hole. He then rotated the first prism around its axis to refract one portion of the supposed spectrum. The refracted light remained red even after it flowed through the second prism. This served to demonstrate that indeed the colors were not being created by the action of the prism. Even more remarkable was that Newton discovered that each color corresponded to a certain angle of refraction, a certain “degree of refrangibility” as he put it. Blue was refracted at the sharpest angle, while red was refracted at a much gentler angle. No refrangibility corresponded to white light.
Newton also observed the formation of interference fringes in thin water film, caused by the reflection of light between a convex surface and an adjacent plane surface, which led to a series of concentric rings of brightness and darkness. Further still, Newton discovered the phenomenon of diffraction by experimenting on hair and knife edges. He passed a narrow beam of light between two knife blades wedged against each other to form a V-shape and observed the diffraction fringes. All of these properties pointed to a wave nature for light. Newton, however, perhaps out of antagonism to Huygens, still fell short of acknowledging the wave theory. He latched onto his corpuscular theory and in order to reach a compromise, proposed that the light particles cause the aether to generate light waves.
The wave theory of light began to gain ground again when the English scientist Thomas Young performed his famous double-slit experiment, where interference patterns were observed when both slits were open, suggesting a wave nature (wavefronts spread out and overlap to form light and dark bands at the screen). He was able to produce the same characteristic diffraction patterns observed when water waves interfere with each other. Unlike Huygens who assumed longitudinal undulations, Young felt that treating light as having both a longitudinal and transverse component would best explain polarization and the interference patterns that he observed.
By the 1860s, the great Scottish physicist James Clerk Maxwell had developed his theory of electromagnetic fields. Maxwell studied electricity and magnetism and his equations showed that if a changing magnetic field could produce a changing electric field, then symmetrically a changing electric field should produce a changing magnetic field. Maxwell’s equations thus predicted an electromagnetic wave combining electrical and magnetic effects that have a transverse nature, being perpendicular to the direction of wave propagation and that could be described by a wave equation. Maxwell’s wave equation indeed showed that the speed of the hypothetical wave in a vacuum would be 2.9986 × 10^8, which was very close to the speed of light. Maxwell also predicted the existence of a family of electromagnetic waves, of which light is one.
” From a long view of the history of mankind, seen from, say, ten thousand years from now, there can be little doubt that the most significant event of the 19th century will be judged as Maxwell’s discovery of the laws of electrodynamics. The American Civil War will pale into provincial insignificance in comparison with this important scientific event of the same decade. ”
– Richard Feynman
Indeed, 9 years after Maxwell’s death, the German physicist Heinrich Hertz was able to experimentally detect electromagnetic waves. His discovery of radio waves showed that they have a similar nature of reflection and refraction and similar properties as light and that they indeed move at the speed of light, confirming Maxwell’s predictions. Mysteries still persisted, in particular, with regards this mysterious ether medium that had long been hypothesized. Since electromagnetic waves were found to be able to travel even in a vacuum, Maxwell’s theories had largely made such a hypothetical medium to be redundant. Experimental attempts, however unsuccessful to detect it, followed along and dominated 19th century physics, in what would pave the way forward for the 20th century revolutions of quantum theory and relativity and our current understanding of light behaviour as a wave-particle duality, acting sometimes as a particle (as shown by Einstein’s photoelectric effect) and sometimes as a wave (as shown by Young’s double-slit experiment).