The two previous Parts told two very different stories about the respective histories of electricity and magnetism. While knowledge of electricity seemed to progress monotonically over the course of time, leading scientists disagreed about the fundamental nature of magnetism for nearly a century. The history of optics took even more twists and turns than magnetism. The greatest minds of the last three centuries wrestled with its most fundamental question—what is light?
We have always been fascinated by optical phenomena, as the figure on the opposite page suggests. Armed with Euclidian geometry, the ancient Greeks investigated optics culminating in Ptolemy’s optical treatise around the first century AD, where he argued that the eye emits a visual flux that probes external objects, called emission theory. Ptolemy also used the idea of refraction to explain whey the moon appears so large when it is near the horizon. Ptolemy’s treatise was unfortunately lost, but not until after somebody translated part of it into Arabic.
Around the turn of the millennium, Ibn al-Haytham (also known as Alhacen and Alhazen) wrote the seven-volume treatise The Book of Optics, where he challenges both Ptolemy’s emission theory of vision and his explanation for the moon illusion. In Alhacen’s view, the eye only receives light and the moon is no larger when it is near the horizon than when it is high in the sky—he was right on both counts. Alhacen also conducted many optical experiments, and developed some of our known laws of optics, including the law of reflection.
A few decades before Peregrinus performed his magnetism experiments, someone translated The Book of Optics from Arabic to Latin, and it became one of the first widely read scientific treatises of the middle ages. In the late sixteenth century, it was published in an anthology called Opticae thesaurus, which included the figure depicting famous optical phenomena of lore, including Noah’s rainbow and Archimedes’s death ray. Now widely published, Alhacen’s ideas permeated Europe, and found their way to the Netherlands, where someone invented a spyglass.
After learning about the Dutch spyglass, Galileo designed and built his own spyglasses, which he pointing toward the sky. Among other things, he observed that Venus goes through the complete range of phases, with it appearing larger when it is a crescent phase, and smaller in a gibbous phase. This completely contradicted Ptolemy’s geocentric model, which required Venus to always be between the Sun and Earth. Galileo, therefore, concluded that Venus must orbit the Sun directly, as the heliocentric model would predict.
A spinning Earth made no physical sense to Galileo’s contemporaries, who argued that a short jump would result in flying many feet to the west, so Galileo reinvented kinematics by proposing his principle of relativity. From that time on, all new physics had to work both terrestrially and universally, thus marrying the fields of physics and astronomy forever.
The development of the telescope, and the new science that it opened up, spurred a renewed interest in optics. This research included two contradictory theories of light: the wave model and the particle model.
In Traité de la Lumière, Christiaan Huygens investigated light by analogy to sound, including accurately calculating the ratio of their speeds. This followed Robert Hooke’s treatise on microscopy, where he briefly discussed light as a wave that propagates through a medium he called the aether.
In Opticks or A Treatise of the Reflections, Refractions, Inflections and Colours of Light, Newton took a completely different tack to explain all the same phenomena—and more. According to Newton, light is not a wave, but rather composed of particles.
Opticks presents Newton’s experiments with light in a manner that is clear, easy to understand, and written in English. The bulk of Opticks contains carefully performed experiments that characterize properties of light. Newton’s most startling experiments were those that investigated color.
Prior to Newton, it was widely believed that colored light contained some kind of impurity. Just as a dye-soaked piece of cloth imparts color to water flowing through it, so would a piece of stained glass lend color to light. Newton shocked the scientific community when he used two prisms to separate light into a rainbow, and then overlapped them again. He describes the experiment, referring to his figure shown, thus:
Then let the Light trajected through them fall upon the Paper MN, distant about 8 or 12 Inches from the Prisms. And the Colours generated by the interior Limits B and C of the two Prisms, will be mingled at PT, and there compound white. For if either Prism be taken away, the Colours made by the other will appear in that Place PT, and when the Prism is restored to its Place again, so that its Colours may there fall upon the Colours of the other, the Mixture of them both will restore the Whiteness.
Since red light refracts least, and blue (violet) light most, Newton theorized that the velocity of a light particle varies with its color. Newton looked to astronomy to verify this hypothesis. If his idea were correct, then blue would be the last color seen during an eclipse of Jupiter’s moons, and red would be the first color observed when the satellite finally emerged from behind the planet. Unfortunately for Newton, no color changes in the eclipses of Jupiter’s moons could be observed by him or any other astronomer.
Astronomy soon did verify another prediction of Newton’s particle model, and at the same time contradicted Huygens’s wave model. In 1725, James Bradley and Samuel Molyneaux serendipitously observed a shift in the positions of stars due to the earth’s motion. Just as rain appears to come toward your windshield, starlight appears to shift in a telescope throughout the year. This aberration of starlight is easily explained using Galilean relativity and the known velocity of light, but only if light is a particle.
Newton’s corpuscular theory of light began to unravel, however, in 1804 when the English polymath Thomas Young presented his famed double-slit experiment. This clearly demonstrated interference patterns of light, just as the Hooke-Huygens theory predicted. When light from a distant source first passes through two open slits and then falls on the screen, there are regions on the screen that are dim but would be bright if only one slit were open. This fact is easily explained by assuming light travels as a wave, so that waves coming from each slit interfere either constructively or destructively.
Young’s wave theory, however, did not immediately take off. Newton had already explained wavelike phenomena as ripples caused in the medium by moving light particles, similar to the ripples caused by rocks hitting water, or the wake produced by a passing boat. More importantly, Young’s wave theory could explain neither Bradley’s observations of stars nor experiments involving the polarization of light.
Augustin-Jean Fresnel successfully explained the results of optical polarization experiments by proposing that, rather than being a longitudinal wave like sound, light waves are transverse like the oscillations of a plucked string. By 1820, Fresnel’s theory had passed every new experimental test, and thus became largely accepted. It still required, however, at least one lucky coincidence—Bradley’s result only makes sense if the aether remains stationary with respect to the solar system.
James Clerk Maxwell finally persuaded the rest of the physics community of the need for the aether when he derived a wave equation from his electromagnetic field equations. By the late nineteenth century, physicists the world over had accepted the aether’s existence as established scientific fact. Maxwell’s theory had miraculously passed every experimental test, especially the exacting experiments of Heinrich Hertz. William Thomson, who has just become Lord Kelvin a year earlier, concludes his preface to the 1893 English translation of Hertz’s book on Electric Waves thusly:
During the fifty-six years which has passed since Faraday first offended physical mathematicians with his curved lines of force, many workers and many thinkers have helped to build up the nineteenth-century school of plenum, one ether for light, heat, electricity, magnetism; and the German and English volumes containing Hertz’s electrical papers, given to the world in the last decade of the century, will be a permanent monument of the splendid consummation now realized.
By building a tabletop interferometer, the American naval officer Albert Michelson tried, and failed, to measure the relative motion of the earth through the aether. Since his data (solid line) show a clear discrepancy from theory (dotted line), Michelson made the bold conclusion we quoted in the overview of the book, that the “hypothesis of a stationary ether is thus shown to be incorrect.” 
Michelson soon moved to a larger university, where he and his new colleague, Edward Morley, built a much more accurate optical interferometer, finally convincing the scientific community that the relative velocity between the earth and the aether is much less than the earth’s orbital velocity.
Recently, modern Michelson interferometers, with fractional errors on the order of one part in nonillion (1030), have finally succeeded in measuring the time lag Michelson was looking for. However they are not interpreted as changes in the speed of light, but rather as gravitational waves caused by coalescing black holes or neutron stars.
By the turn of the century the problems with the theory of light came to the fore. In conjunction with the 1904 world’s fair in St. Louis, the French physicist, J. Henri Poincaré, gave a lecture so brilliant we reproduce in full in Appendix C. In his section on the principle of relativity, and its recent breakdown, he says this about Michelson’s work:
And then experiment, too, has taken upon itself to refute this interpretation of the principle of relativity; all the attempts to measure the velocity of the earth relative to the aether have led to negative results. Herein experimental physics has been more faithful to the principle than mathematical physics; the theorists would have dispensed with it readily in order to harmonize the other general points of view; but experimentation has insisted on confirming it. Methods were diversified; finally Michelson carried precision to its utmost limits; nothing came of it. It is precisely to overcome this stubbornness that today mathematicians are forced to employ all their ingenuity.
Their task was not easy, and if Lorentz has succeeded, it is only by an accumulation of hypotheses. The most ingenious idea is that of local time.
Let us imagine two observers, located at signal stations A and B, who wish to regulate their watches by means of optical signals. They exchange signals, but as they know that the transmission of light is not instantaneous, they are careful to cross them.
And, indeed, they mark the same hour at the same physical instant, but only if the two stations are stationary. Otherwise, the time of transmission will not be the same in the two directions, since the station A, for example, goes to meet the disturbance emanating from B, whereas station B flees before the disturbance emanating from A.
Watches regulated in this way, therefore, will not mark the true time; they will mark what might be called the local time, so that one will gain on the other. It matters little, since we have no means of perceiving it. All the phenomena which take place at A, for example, will be behind time, but all just the same amount, and the observer will not notice it since his watch is also behind time; thus, in accordance with the principle of relativity he will have no means of ascertaining whether he is at rest or in absolute motion.
Unfortunately this is not sufficient; additional hypotheses are necessary. We must admit that the moving bodies undergo a uniform contraction in the direction of the motion. One of the diameters of the earth, for example, is shortened by 1/200000000 as a result of our planet’s motion, whereas the other diameter preserves its normal length. Thus we find the last minute differences accounted for.
It was around this same time that Albert Einstein led a book club where they read Poincaré. A year later, you can see Poincaré’s influence in Einstein’s 1905 paper On the Electrodynamics of Moving Bodies:
Like every other electrodynamics, the theory to be developed is based on the kinematics of the rigid body, since assertions of each and any theory concern the relations between rigid bodies (coordinate systems), clocks, and electromagnetic processes.
The connection that Einstein made was simple. Keep Galileo’s principle of relativity and Maxwell’s electrodynamics, but modify basic kinematics in order to make the union work. While the most startling results have to do with interpretations of time and length, the beauty of special relativity comes in its reinterpretation of electrodynamics.
With Einstein’s theory of relativity, there is no longer a need for an aether. But with no medium, why does light act as an electromagnetic wave? What is it fundamentally? Does a field have any real meaning other than a way to put off harder physical questions, and advert action at a distance?
To address these question, Einstein made the heuristic hypothesis that Newton was right after all, at least in principle, about light. This new theory of light, as a particle that follows super-relativistic kinematics, seemed to work. Einstein published his particle model of light just months before his special theory of relativity, and it was the only one of his papers that he conceded was truly “revolutionary.”
What is the correct paradigm? Is light fundamentally a particle that often acts like a wave, or is it a wave that often acts as a particle? What is the solution to spooky action at a distance? Is the universe filled with some medium, be it called the aether, dark matter, dark energy, or something else? Some of these questions are still open. Keep them in mind as you study not only electrodynamics, but other scientific and philosophical fields as well.
 See A. Mark Smith, “What is the History of Medieval Optics Really About?” Proceedings of the American Philosophical Society (2004) 148, No. 2: 180-94. Also see Jim Al-Khalili, “In retrospect: Book of Optics” Nature (2015) 518, 164-165.
 Federico Risnero, Opticae thesaurus (Basile AE per Episcopios, 1572).
The Sicilian city-state of Syracuse famously fought off a vast Roman army for two years, primarily because of the ingenuity of the old Archimedes. While most of his inventions were mechanical in nature, Archimedes supposedly instructed the troops to use their shiny bronze shields to reflect sunlight on the attacking Roman ships—setting them ablaze. The television show Mythbusters (29-September-2004) found the “Archimedes Death Ray” to be implausible (“Busted”). However, the next year MIT students enrolled in Engineering 2.009 succeeded, so Mythbusters revisited the issue (25-January-2006), but again found it unlikely. They revisited the question four years later (8-December-2010), yet again with a null result.
 This insight did not originate with Galileo, or even with Copernicus. Centuries before Ptolemy, Aristarchus of Samos proposed a heliocentric universe. Unfortunately his treatise did not survive, but Archimedes of Syracuse discusses it in The Sand Reckoner.
 C. Huygens, Treatise on Light, (1690), Translated by S. P. Thompson, Project Gutenberg E-book number 14725. The figure is from page 17 of the same source.
 R. Hooke, Micrographia: Some Physiological Descriptions of Minute Bodies Made by Magnifying Glasses with Observations and Inquiries Thereupon, (1665), London, Printed by Jo. Martyn, and Ja. Allestry, Printers to the Royal Society, Project Gutenberg E-book number 15491.
 Newton actually wrote three English editions (1704, 1717, and 1730), and a single Latin edition in 1706.
 Isaac Newton, Opticks or A Treatise on the Reflections, Refractions, Inflections and Colors of Light, 1730 Edition, Book One, Part II, Experiment 13.
 Alan Shapiro, “Newton’s optics and atomism,” in The Cambridge Companion to Newton, ed. I. Bernard Cohen and George Smith (Cambridge University Press, 2004), pp. 236-7.
 All measures of the speed of light at the time depended, in the same way, on the assumed length of one astronomical unit, so the uncertainty in the AU was not a problem.
 Thomas Young, “The Bakerian Lecture: Experiments and calculations relative to physical optics,” Philosophical Transactions of the Royal Society of London (Royal Society of London) 94 (1804), 1–16.
 Young, Thomas, “The Bakerian Lecture: Experiments and calculations relative to physical optics,” Philosophical Transactions of the Royal Society of London (Royal Society of London) 94 (1804), pp. 12-13. See also, E.T. Whittaker, A History of the Theories of Aether and Electricity (Dublin: Dublin University Press, 1910), p. 115.
 H. Hertz, Electric Waves, trans. D. Jones (London: MacMillan and Co., 1893), p. xv. (see Appendix B)
 Albert A. Michelson, “The Relative Motion of the Earth and the Luminiferous Ether”, American Journal of Science, 22 (1881), 120-129.
 Albert A. Michelson & Edward W. Morley, “On the Relative Motion of the Earth and the Luminiferous Ether”, American Journal of Science 34 (1887), 333–345.
 A. Einstein, “On the Electrodynamics of Moving Bodies,” Annalen der Physik 17 (1905): 891-921, Translated by Anna Beck, in The Collected Papers of Albert Einstein, Volume 2, The Swiss Years, 1900-1909. John Stachel, ed. (Princeton: Princeton University Press). English Translation, ©1989 by the Hebrew University of Jerusalem.