A post inspired by work I’m doing on my next book, on the history of invisibility! Also will help me get my thoughts in order to write the book chapter.

At the turn of the nineteenth century, humanity’s understanding of the nature of light underwent a dramatic transformation. For about a century, the study of optics had been dominated by the views of Isaac Newton, who published his classic work on the subject, Opticks, in 1704. Newton did rigorous experiments, testing the properties of light in every way imaginable in his era, and concluded from his work that light consists of a stream of particles. Newton’s work seemed to put to rest an argument that had raged in his time — is light a stream of tiny particles, or a wave, like water and sound? In 1800, however, the British scientist Thomas Young published the first of several papers arguing that light does, in fact, have wavelike properties, and his research would be the start of a new era of wave optics that continues (with some important modifications) to this day.

I’m not sure how many people are aware of the work of Young. I think most scientists have heard about his most famous contribution, now known as Young’s double-slit experiment, but even most of them may not be aware of the fascinating story of how Young came to his conclusions. This post is an attempt to rectify this.

Though he had diverse scholarly interests, Young initially focused on a career as a medical doctor. He was inspired by his great uncle Richard Brocklesby, a physician of great renown in London who had also treated Young when he fell seriously ill in his teens. Furthermore, by pursuing the medical path, Young was guaranteed an inheritance from his uncle, and financial security with it. So in 1793, Young became a student at the venerable St. Bartholomew’s Hospital in London, established in the year 1123. For some context, St. Bartholomew’s was founded only 24 years after the First Crusade ended.

Even in medicine, Young found himself drawn into questions of optics. In his studies of anatomy, he had learned of an unsolved question in vision: how does the eye of a living creature adjust, or accommodate, to produce clear images of objects at any distance? Through dissection of an ox eye, Young came to the conclusion that the crystalline lens of the eye has its shape distorted by the actions of muscles, changing the focusing properties of the lens accordingly. Young produced a paper on the subject, and presented it to the prestigious Royal Society of London on May 30, 1793. The paper was so well-received that Young was made a Fellow of the Royal Society the following year, at age 21.

Young’s paper, “Observations on vision,” met quickly with controversy and condemnation. Research rivals argued that his results were incorrect, as they had not observed any deformation of the lens of the eye in their own investigations. Adding to Young’s troubles, a famous surgeon named John Hunter claimed that Young had overheard and copied his ideas on the subject. Though the plagiarism charge was quickly dismissed, Young ended up renouncing his own work on the human eye for a time, deferring to the experts in the field. This retraction would cause him additional troubles in the future, though science has shown that his explanation of the eye was completely correct.

As part of a plan to give him a well-rounded medical education, Young proceeded to Edinburgh to continue his studies, and from there went to Göttingen to earn a degree. He did not stay in Göttingen for long, however: he and his uncle had misunderstood the rules for practicing medicine in London, which required two years of residence in a London medical school in order to qualify to become a Fellow of the Royal College of Physicians. Upon learning this, Young hastened to finish his degree in Göttingen, and ended up staying for a total of only nine months.

As part of the degree requirements in Göttingen, Young had to give a lecture related to medicine. Young chose as his topic the working of the human voice. This required him to study the properties of sound waves, and during the course of his investigations he was struck by the similarity of phenomena involving sound and those involving light. Though researchers had long concluded that light does not possess wave properties, the similarities between light and sound waves seemed too strong to be coincidental, and drove Young to explore the possibility that light is in fact a wave.

After Göttingen, Young entered Emmanuel College in Cambridge for the last stage of his training, and finished two years later, in the autumn of 1799. After that, he set himself up in private medical practice in London, as planned. Such practices were slow to grow and accumulate patients in that era, however, and left Young plenty of free time to ponder those scientific questions that had puzzled him for years.

He began his work with a number of essays on a range of scientific topics, including the properties of sound waves, that appeared in the British Magazine under the pseudonym “The Leptologist.” Having been embarrassed in public before while sharing his scientific views, he appears to have used the pseudonym as a way to wade back into debate without risking his reputation. He officially reentered the scientific debate in a letter that was published by the Royal Society in January of 1800, “Outlines of Experiments and Inquiries respecting Sound and Light.” The letter is primarily an analysis of sound waves and their behavior, but Young also noted the similarities between sound and light, a hint of his work to come. Later that year, Young would published “On the mechanism of the eye” in the Philosophical Transactions of the Royal Society, a renewed defense of his hypothesis on the properties of the lens of the eye.

To further his renewed scientific activity, Young accepted a job in 1801 as a Professor of Natural Philosophy at the Royal Institution, an organization that had been founded only two years earlier to foster scientific education and research. With this new role, he focused his energies on studying the properties of sound and their remarkable similarities to light.

To understand what Young saw, we now need to spend a little time discussing what, in fact, a wave is! In technical terms, we may describe a wave as an oscillatory motion of “something” that transports energy from one place to another but does not transport the “something” itself. Waves in water are the simplest example to visualize, as they move slowly enough to be seen with the eye and also possess all of the properties of other waves, like sound waves and light waves.

When a rock is dropped into a pond, ripples spread out from the point of impact, usually manifesting as a succession of “up” and “down” regions where the water level is higher or lower than normal, respectively. These ripples can travel great distances along the surface of the water before dissipating, and can disturb objects on the water’s surface, like leaves (or waterfowl). This ability to move distant objects demonstrates that the waves carry energy. But there is no net flow of water away from the place the rock is dropped itself: the water level (the “something” for water waves) locally goes up or down, but the water itself does not move away from the point of impact. This behavior is distinct from the motion of water in a stream, where the water actually flows downstream en masse and eventually empties into a lake or ocean.

Another example of a wave is a vibration on a stretched piece of string or elastic, such as a string on a guitar (or a classic Slinky toy). When the string is plucked, that disturbance is carried along the length of the string. The waves carry energy along the string, but the string itself stay in one place, firmly attached to the guitar.

The simplest form of wave is one that vibrates as shown in the picture below. It consists of a simple repeating up and down motion, mathematically taking on the form of a sine wave from trigonometry. In optics, for reasons that will become clear, these are called monochromatic (“single color”) waves.

If we look at the string at a single point along its length, and watch what it does in time, we see that the wave will alternately move upwards and downwards. This is analogous to sitting in a docked boat and feeling the boat rise and fall beneath you as waves pass by. The amount of time between peaks is called the period of the wave and the inverse of this is called the frequency, which tells you how many peaks you feel every second.

If we take a snapshot of the string at a single point in time, we see a similar picture: along its length, it has alternating upward and downward wiggles. The spacing between the peaks in space is called the wavelength, as it represents the physical length of a single up-and-down cycle of the wave.

For sound waves, the “something” that is waving is the density of the air molecules. These alternating regions of high and low density travel through the air, and cause vibrations on our eardrums that we perceive as sound. Again, this is a motion distinct from the transport of air molecules through space, which we perceive as a wind or a breeze. A sketch of air molecule density at an instant of time is shown below. In music, “middle C” has a frequency of 261 cycles per second, which corresponds to a wavelength of 132 centimeters. Higher notes have higher frequencies and correspondingly shorter wavelengths.

One particular property of sound waves drew Young’s attention, a phenomenon known today as resonance. When a sound is created in an enclosed space, waves with wavelengths that fit perfectly into that space will build upon themselves, becoming louder. If you’ve ever thought your singing sounds better in the shower, you’ve experienced the resonance of sound waves: the walls of the shower form an enclosed space that causes particular tones to be enhanced. A resonant wave doesn’t actually move, but oscillates in place in its confined space, and is referred to as a standing wave.

A simple example of resonance is found in the operation of pipe organs. An organ pipe with openings at both ends naturally fits waves inside it that have a maximum or minimum at each end. This means that the lowest tone that a pipe will produce will have a half-wavelength fitting inside the pipe, as illustrated below. A wave of this length will build up rapidly over time, producing a loud, clear tone. Waves of shorter wavelengths will also resonate in the pipe, provided they too are of such a wavelength that there is a maximum or minimum at each end. This means that there are a number of different wavelengths that can resonate in the pipe; the longest wavelength possible is called the fundamental tone, and the shorter wavelengths are called the harmonics. The fundamental tone is usually the loudest, and defines the distinctive pitch of the note. The combination of the fundamental tone and the harmonics is what gives an instrument its distinctive sound.

This same phenomenon is the basis for most wind instruments: a musician playing a recorder or flute, for example, changes the note they are playing by opening or closing holes in the instrument, effectively making it a shorter or longer hollow tube.

In the resonant behavior of organ pipes, Young saw an explanation for a phenomenon discussed in detail by Isaac Newton at length in his Opticks, and which was later named after him: Newton’s rings. Newton placed a glass lens with a very shallow curvature on top of a flat piece of glass, and observed colorful rings emanating from the center outward, as shown below.

Newton interpreted the colored rings as the result of a complicated process of reflection and refraction of light as it bounces back and forth inside the gap between the lens and the glass plate. Young, however, saw something else: if light is a wave, then the gap, which increases in thickness as one moves away from the center, could serve as a sequence of organ pipes. The colors seen by Newton would therefore represent light waves resonating between the pieces of glass in regions of different thickness. Each color of light would represent a wave with a different wavelength and frequency. Looking at a classical pipe organ, and visualizing little pipes in the gaps in Newton’s rings experiment, one can imagine how Young came to his inspired idea.

Young first hinted at these possibilities in his letter, “Outlines of Experiments and Inquiries respecting Sound and Light.” In the short section on light, “Of the analogy between light and sound,” he presented his analogy between Newton’s rings and pipe organs, and reintroduced the idea, first given by the mathematician Leonhard Euler, that the colors of light are the visible manifestations of the light’s frequency. He also addressed a common misconception about waves that had been propagated since the time of Newton, and was part of Newton’s argument against the wave nature of light: that waves, when generated, spread equally in all directions. Newton argued that waves, like those created by dropping a rock into a pond, spread out in all directions. Light, however, was observed to be highly directional, as when sunlight peeks through the clouds, creating what are technically known as crepuscular rays but often called “god rays.”

Young argued that Newton was simply mistaken: though sound waves do spread out a lot, they can also be quite directional, as ordinary experience indicates:

It is well known, that if a person calls to another with a speaking trumpet, he points it towards the place where his hearer stands: and I am assured by a very respectable Member of the Royal Society, that the report of a cannon appears many times louder to a person towards whom it is fired, than to one placed in a contrary direction.

There was no reason, then, to think that light waves could travel with even less spreading than sound waves. Light waves would then not even need “pipes” like an organ in order to produce the colors of Newton’s rings.

Young continued his experiments, and gave a prestigious Bakerian Lecture to the Royal Society on November 12, 1801, “On the Theory of Light and Colours.” In it, he provided a comprehensive theory of the wave properties of light. Among his achievements: he estimated the wavelength and frequency of the different colors of light, using Newton’s original measurements of the gap thicknesses in Newton’s rings. For red light, for example, he calculated a wavelength of 0.675 billionths of a meter, and a frequency of 463 million-million oscillations per second. For blue light, he calculated a wavelength of 0.5 billionths of a meter, and a frequency of 629 million-million oscillations per second. These numbers are quite reasonable by modern standards considering that the terms “red” and “blue” refer to a broad range of frequencies and wavelengths.

It was not just courtesy that motivated Young to use Newton’s own measurements and experimental observations: he well knew that criticizing the legendary man’s work could provoke a strong backlash against him, the young upstart. Young took great pains in his paper to point out how Newton’s own experiments and hypotheses could be used to support a wave theory of light. Young was, in essence, attempting to give Newton partial credit for the discovery.

In the myriad hypotheses Young introduced in his paper at the time, it would have been easy to overlook one of these, in which he considers what happens when waves cross paths with each other. He introduces what would eventually be called the law of interference, one of the most important properties of waves, as follows:

When two Undulations, from different Origins, coincide either perfectly or very nearly in Direction, their joint effect is a Combination of the Motions belonging to each.

In terms of water waves, we may imagine two waves created separately, intersecting at a point. If both waves are moving upward at the same time, their actions will combine to make an even bigger wave; if one wave is moving up and the other wave is moving down, their actions will at least partially cancel each other out, leading to a smaller wave than either contribution. The former case is now called constructive interference, while the latter case is called destructive interference.

This is illustrated below for two square waves on a string, propagating towards each other. If both pulses consist of an “upward” wiggle, they will combine when they intersect; if one is “up” and one is “down,” they will cancel. It is to be noted that the waves don’t destroy each other — they carry on their separate ways once they pass each other. In a word, they “interfere” with each other.

Young used his law of interference to explain a number of optical systems where colors unexpectedly appear, including Newton’s rings as well as parallel scratches on polished surfaces, which today would be referred to as a diffraction grating. In a follow-up paper to the Royal Society which was presented in July 1802, Young introduced a new observation: bands of colors appearing when light goes around a fine fiber or hair. Young cut a small hole in a piece of cardboard, and fixed the fiber across the center of the hole. Looking at a distant light source through the hole, he saw colored bands of light on either side of the fiber, parallel to it. He interpreted these colors as arising from interference between light waves that pass on opposite sides of the fiber. Because different colors have different wavelengths, the points in space where their “ups” and “downs” meet are different, resulting in different colors in different locations.

Young continued his experimentation, and in November of 1803 presented his strongest case for the interference of light in a Bakerian Lecture titled “Experiments and Calculations relative to Physical Optics.” It would be the first rough demonstration of what later became known as Young’s double slit experiment or Young’s two-pinhole experiment. In this rough implementation, Young poked a small hole in a window shutter to allow a thin beam of sunlight to enter his room. In the path of that beam, he placed a thin piece of card, one thirtieth of an inch thick, dividing the beam into two parts, each of which spreads into the path of the other. The combined light wave is then projected onto a screen some distance beyond, allowing the multiple colored fringes to be seen in the shadow cast by the card. This, Young felt, was conclusive proof of the wave nature of light.

Young refined his experiment over time. The thick card was replaced with “two very small holes or slits” in his later 1807 book A Course of Lectures on Natural Philosophy and the Mechanical Arts. In this arrangement, the pattern on the screen consists of only the two contributions from the small holes. In this book, Young provided a beautiful picture of how the waves from two holes produce interference, as shown below.

In this picture, A and B represent the two pinholes. The light waves spread out as circular ripples from the holes, and eventually overlap. If we consider the white regions of each ripple as the “up” part of the waves and the dark lines as the “down” part of the waves, one can see that lines form where the dark lines of one source cross the white regions of the other source. These become regions of complete destructive interference, and form lines that end at points C, D, E and F on an observation screen.

A modern conception of Young’s experiment is illustrated below. If a light source consisting of a single color is used to illuminate the holes, the pattern consists of a sequence of light and dark lines — the regions of constructive interference and destructive interference.

Young went even further with his novel law of interference, and used it to explain the phenomenon of supernumerary rainbows. It had long been observed that rainbows sometimes possess extra faint bow lines just inside the main rainbow, but a satisfactory explanation for them had not been given. Young explained such supernumerary rainbows as arising from wave interference within individual raindrops. The bows only appear when the raindrops are all roughly the same size, and therefore produce the same size and shape supernumerary bows.

Young made one other significant observation of particular interest. In 1800, the German astronomer William Herschel had discovered infrared radiation, invisible heat radiation lying just outside the red side of the visible spectrum. The following year, the chemist Johann Wilhelm Ritter discovered ultraviolet radiation, lying outside the violet side of the visible spectrum and detectable through chemical changes. Young reproduced his Newton’s rings experiments with ultraviolet radiation, and through the use of paper dipped in a solution of nitrate of silver, acting like photograph paper, showed that ultraviolet light also produces interference. He showed that the wave properties of light also extended out beyond one side of the visible spectrum, and speculated that the same would be true for infrared light.

It appears that Young’s research was at least politely received. He had, as we have seen, been asked to give three Bakerian Lectures within a four year period, two on his theory of light and one on the mechanism of the eye. But this non-hostile reception did not apparently result in widespread acceptance of his ideas.

Young worked hard to avoid controversy by giving heavy credit to Newton in his work, but controversy found him nevertheless from an unexpected source — his own injudicious earlier writings. In one of his essays as The Leptologist, Young somewhat flippantly criticized “a young gentleman in Edinburgh” for rediscovering things that had been well-known for years. This “young gentleman” was Henry Peter Brougham, who like Young had taken an interest in optics at an early age and had written several papers on the subject for the Royal Society (he would be made a Fellow in 1803). Unlike Young, however, Brougham was a devoted disciple of Newton, and the combination of perceived attacks by Young on himself and Newton was too much to bear. In 1802, Brougham founded the magazine the Edinburgh Review, and in 1803 Brougham anonymously launched a series of vicious attacks on Young’s optical work in the magazine.

On Young’s “Theory of Light and Colours,” for example, Brougham began his attack thusly,

As this paper contains nothing which deserves the name, either of experiment or discovery , and as it is in fact destitute of every species of merit, we should have allowed it to pass among the multitude of those articles which must always find admittance into the collections of a Society which is pledged to publish two or three volumes every year.

On Young’s 1802 paper discussing the colors produced around fibers, Brougham appears almost gleeful as he accuses Young of rediscovering known facts, something that Young had earlier accused him of:

We are sorry to find that Dr Young is by no means more successful at making observations and experiments, than in forming systems. The new case of colours which he affects to have discovered, has been observed a thousand times; and he has only the merit of giving an absurd and contradictory explanation of it.

The attacks, though in fact largely based on errors and misunderstandings, definitely wounded Young’s pride. He responded by publishing his own short pamphlet of rebuttals in 1804, “A Reply to the Animadversions of the Edinburgh Reviewers.” From the very first words, it is clear that the argument between the men had gone far beyond a simple scientific dispute:

A Man who has a proper regard for the dignity of his own character, although his sensibility may sometimes be awakened by the unjust attacks of interested malevolence, will esteem it in general more advisable to bear, in silence, the temporary effects of a short lived injury, than to suffer his own pursuits to be interrupted, in making an effort to repel the invective, and to punish the aggressor. But it is possible that art and malice may be so insidiously combined, as to give to the grossest misrepresentations the semblance of justice and candor…

Young defended not only his scientific ideas, but attacks on his character. Young had retracted, then resubmitted, his ideas on the behavior of the eye as new information came in, and Brougham used this to characterize Young as a man confused about science. Young in return laid out a detailed history of the entire controversy, explaining why he had taken the actions that he had. In the end, though, Young seemed worn down by the controversy that had dogged his scientific efforts from the beginning, and announced his intention to return to medicine,

With this work, my pursuit of general science will terminate: henceforwards I have resolved to confine my studies and my pen to medical subjects only. For the talents which God has not given me, I am not responsible, but those which I possess, I have hitherto cultivated and employed as diligently as my opportunities have allowed me to do; and I shall continue to apply them with assiduity, and in tranquility, to that profession which has constantly been the ultimate object of all my labours.

And for a while, other than the publication of his Course of Lectures on Natural Philosophy, Young did return to his medical studies. He had resigned his post at the Royal Institution in 1803, and finally earned the accreditation he needed to become a physician at St. George’s Hospital in 1811.

The wave theory would again seem to go into hibernation after Young’s efforts, but behind the scenes, others were following in his footsteps and developing a strong mathematical theory to support his observations. On March 17, 1817, the French Académie des Sciences announced that diffraction would be the topic for the biannual physics prize to be awarded in 1819. These Grand Prix events were intended to spur innovation in physics, and drive solutions to unanswered questions. One of the entrants was a 28 year old civil engineer named Augustin-Jean Fresnel, who presented a comprehensive wave theory of light, backed up back experimental work.

Fresnel had, like Young, long been interested in questions of optics. He also found himself with leisure time to explore these questions, but from a very different circumstance. The Emperor Napoleon, who had been exiled to the island of Elba in 1814, escaped in February of 1815 with an eye towards reclaiming his throne. Fresnel was one of those who joined the royalist resistance against Napoleon, though he fell ill and was unable to participate directly. When Napoleon reclaimed his throne, Fresnel found himself ostracized, and he was essentially given house arrest that he spent at his mother’s house. With this free time, he was spurred to start his series of optical investigations, which continued even after Napoleon was again dethroned in mid-1815 and Fresnel’s job was restored.

Fresnel had long found himself fascinated by the possibility that light is a wave. In the course of his investigations, he struck up a correspondence with the French physicist François Arago, who directed him to the work of Thomas Young and others. Arago pointed out that Fresnel had inadvertently retraced many of the steps that Young had already taken, but was generally encouraging of the young engineer. When the Académie des Sciences announced diffraction as the 1819 problem, Arago encouraged Fresnel to enter the competition.

The Prize Committee contained some of the greatest minds in physics of the era: François Arago, Pierre-Simon Laplace, Siméon Denis Poisson, and Joseph Louis Gay-Lussac. When Fresnel submitted his prize entry on July 29, 1818, the committee gave it a thorough investigation. Poisson, one of the supporters of the particle theory of light, noted a curious implication of Fresnel’s new wave theory, that arises when one calculates how light diffracts around an opaque circular disk. According to the law of interference, Poisson showed, one would expect that there should be a bright line right in the center of the disk on the shadowed side, as the waves diffracting from all edges of the disk should constructively interfere on the axis line of the disk. For proponents of the particle theory of light, this was an absurd conclusion — how could light appear miraculously right in the middle of a shadow? But Arago went and did the experiment, and confirmed that the bright line is there, right as Fresnel’s theory predicted; this spot is now often referred to as the “Arago spot.”

Fresnel was awarded the Grand Prix for his work, as was announced on March 15, 1819 at a meeting of the Académie. This award alone did not convince people that light is a wave, but might be considered a turning point for the theory. The Arago spot showed that the wave theory could not only explain existing experimental observations, but predict new phenomena. This drove other researchers to explore the possibilities, and new evidence to support the wave theory piled up, eventually silencing even the strongest proponents of Newton’s corpuscular theory. The era of the wave theory of light had begun.

Thomas Young did not stay silent during all of this. In 1817, he started correspondence with none other than François Arago, explaining how other curious properties of light could be explained by the wave theory. His fundamental contributions to the theory of light became recognized worldwide: in the 1820s, he was elected to the American, French, Swedish, and Netherlands science academies.

True to his early polymath tendencies, Young spent his later years exploring a variety of subjects. He wrote a number of influential books on medicine, and wrote articles for Encyclopædia Britannica on a variety of subjects. He served on a number of public committees and commissions, including one to study the possible dangers of introducing gas lighting into London. He also exercised his talent in languages to work on deciphering Egyptian hieroglyphics, which were still cryptic at the time.

When Young died at the age of 55, he was recognized as a singular genius of his time. A white marble tablet was erected at Westminster Abbey in 1834, with an epitaph by Hudson Gurney, the family friend who Young had tutored so many years before. The epitaph reads:

Sacred to the memory of Thomas Young, M.D., Fellow and Foreign Secretary of the Royal Society Member of the National Institute of France; a man alike eminent in almost every department of human learning. Patient of unintermitted labour, endowed with the faculty of intuitive perception, who, bringing an equal mastery to the most abstruse investigations of letters and of science, first established the undulatory theory of light, and first penetrated the obscurity which had veiled for ages the hieroglyphs of Egypt. Endeared to his friends by his domestic virtues, honoured by the World for his unrivalled acquirements, he died in the hopes of the Resurrection of the just. — Born at Milverton, in Somersetshire, 13 June 1773. Died in Park Square, London, 10 May 1829, in the 56th year of his age.