Spie Press BookOptical Anecdotes
|Format||Member Price||Non-Member Price|
|GOOD NEWS! Your organization subscribes to the SPIE Digital Library. You may be able to download this paper for free.||Check Access|
As man emerged from his primeval state, he became conscious of the formation of shadows, the color of the daytime sky, the spectacle of the rainbow, and the motions of the stars at night and the sun in the day. Thus, he began his awareness of light. Initially, he could seek to discern the nature of light only by noting its rectilinear motion. But this soon led to observations which tended to clarify the understanding of mechanisms responsible for these phenomena.
The invention of the telescope paved the way for a dramatic impact on science. The plaudits for this turn of events, however, are not to be bestowed on the inventor of the telescope, but on Galileo Galilei. it was his remarkable observations and sagacious interpretations that introduced a new perspective of scientific method.
By the Ninth Century, towns such as Venice and Naples (later Pisa and Genoa, as well) were carrying on trade throughout the eastern Mediterranean. The Crusades began at the end of the eleventh century. Marco Polo visited the Mogul Empire of Kublai Khan in the thirteenth century. These peripatetic adventures brought men into closer contact with their neighbors. At the same time, a renaissance of learning spread through Europe. Moreover, these contacts enabled Europe to become aware of Arabian discoveries and those Hellenistic achievements maintained in their repository.
Why did science begin to bloom in the seventeenth century? There are many ways to answer this simple question. The intellectual reawakening begun with the Renaissance was broadening with a substantial impact on science. Shakespeare lived from 1564 to 1616. In France, Jean Racine (1639 to 1699) moved passions with his writings. John Milton (1608 to 1674) espoused the republican view in England. Francis Bacon (1561 to 1626) contributed to both philosophy and science, and attained political eminence.
By the middle of the Seventeenth Century, observations of most of the manifestations of light had been made. The refractive properties had been relegated to a mathematical formula. The finite velocity had been measured. Diffraction and double refraction had been observed, although neither was understood. Telescopes and microscopes were being constructed with a variety of designs. Moreover, these designs could be based upon knowledge of the passage of light through such systems. The use of these instruments enabled man to discern that the Copernican system made much more sense than a geocentric system.
When we consider how the understanding of the nature of light was being established prior to the mid-eighteenth century, we are likely to think only of the contributions of such distinguished individuals as Plato, Ibn al-Haitham, Galileo, Huygens, and Newton. Certainly, it was necessary to extend and verify their work, and numerous workers were needed to provide this, but the impact of their efforts generally was incidental to the mainstream of the development of optical knowledge. Accordingly, it hardly seems possible that an uneducated sheepherder could contribute to the erudition of optical knowledge in the mid-eighteenth century. But it happened.
Patents are a curious matter. Doubtless, invention prospers with the incentive proferred by a patent. Technology is certainly dependent upon invention and, therefore, in debt to the patent system, Since scientific advances often require improved technology to supply requisite data, it may be argued that science is also in debt to the patent system. Yet, the discoveries made in scientific areas are usually published in the open literature for all to share. In fact, the general character of a scientist’s nature is to publicize his achievements. Seldom does a scientist hide his discovery in anticipation of personal gain. Exceptions exist, of course, and a consideration of the discovery of the achromatic lens might be cited as a case in point. The achromatic lens, which Newton had said could not be made, was perfected in 1733 by the barrister Chester Moor Hall. He practiced his art in secrecy and for nearly a quarter century prevented an invasion of his concealed capability. By 1758, John Dollond had found out the mystery, patented the process, and presented an account to the Royal Society.
When one advocates a viewpoint, he generally proceeds by bringing to bear those aspects of the matter which will reinforce his Position. In these anecdotes, the viewpoint being considered is the evolution in our understanding of the nature of light. It is, therefore, appropriate to consider how men discerned those properties of light that led to an understanding. It would be remiss of me, however, to suggest that observations followed in an orderly manner until the nature of light became evident. Indeed, there are many manifestations of light that tend to obscure rather than to illuminate (if you will pardon my selection of that word) our understanding of light. It is of interest to consider some of the observations that tended to obscure our understanding.
Science, by the end of the Eighteenth Century, was making remarkable progress. Nevertheless, as investigations opened one door to provide a glimpse of physical reality, other doors were exposed to present a conflicting interpretation of the significance of the vista pieced together from this complexity of views. Certainly, this was true of the search to understand the nature of light. While Plato’s ocular beams were no longer considered a reasonable hypothesis, light was considered by some to be the transmission of small particles—of corpuscles—while others advocated some sort of wave motion. In either event, light was responsible for vision and thus was correlated with this sensory response. One could hardly conceive at the end of the eighteenth century of invisible light. But in the year 1800 such was reported, leading to a new comprehension of light itself.
The discovery of “invisible light” by William Herschel in 1800 not only incited contradictory responses from his peers, but caused Herschel to doubt its significance. Two decades earlier, the tenor of the times was such that his discovery of a new planet made considerable impact. But now scientists had become accustomed to the revelations of additional discoveries. The concepts of science were undergoing frequent revision. Nevertheless, invisible light seemed a contradiction of terms.
Double refraction in a crystal had been observed as early as 1669 by the Danish naturalist Erasmus Bartholin (1625 to 1698). Using a crystal of Iceland spar he showed that an incident beam of light was split into two rays by what he called ordinary and extraordinary refraction. He could give no theoretical explanation for this. Christiaan Huygens (1629 to 1695) advocated a wave theory of light that would explain double refraction as the formation of spherical and spheroidal waves. However, since Huygens considered the waves to be longitudinal rather than transverse oscillations, his wave theory could not account for many of the other observations of light phenomena, hence was not generally accepted. The readily observed phenomena of double refraction thus remained an enigma into the eighteenth century.
David Brewster (1781 to 1868) is known to every student of optics for his contributions to our knowledge of polarized light, particularly through the study of Brewster’s angle. His publications include over 300 technical papers, several books, and numerous reports in encyclopedias, etc. He helped found the British Association for the Advancement of Science and was a fellow of the Royal Society from which he was awarded the Copley medal, the Rumford medal, and six other Royal medals. He was a corresponding member of the French Institute, from which he was honored with prizes for his contributions to optics, and was also president of the Royal Society of Edinburgh. Beyond this, he also achieved many other distinctions. Yet, Brewster’s daughter, two years after his death, regarded that his invention of the kaleidoscope, “though of little practical advantage, spread his name far and near, from schoolboy to statesman, from peasant to philosopher, more surely and lastingly than his many noble and useful inventions.”
When Thomas Young (1773 to 1829) discovered interference as a result of his “double-slit” experiment early in the nineteenth century, he could obtain no better than an estimate of the wavelengths of light. He had no “benchmarks” to guide him and could only conclude, “the undulations constituting the extreme red light must be supposed to be, in air, about one 36-thousandths of an inch, and those of the extreme violet about one 60-thousandths.” While these values are quite accurate, they permit but limited quantitative calculations of optical phenomena. Within about twenty years, however, means were found to provide the benchmarks necessary for quantitative determinations.
When Josef Von Fraunhofer (1787 to 1826) discovered dark lines in the solar spectrum in 1814, he opened an entirely new field of optical investigations. Appropriately, it began with Fraunhofer’s continued investigations of the spectral characteristics of the light emitted by various lamps. The paths to the use of spectroscopy as an investigative tool, however, had several bridges yet to cross. It is of interest to review some of these.
The concept of an ether to convey light probably seems strange and unnecessary to an optical engineer today. However, until well into the nineteenth century it seemed to be necessary to depend on a hypothetical fluid to convey all physical substances. Sound, of course, is conveyed by the air. Heat was considered as a substance called “caloric.” Fire was conceived as “phlogiston.” Electricity and magnetic fields were carried by “effluvia.” Accordingly, the concept of an ether to convey the vibrations of light seemed to be quite reasonable.
The pioneering spirit with which America was founded endowed the country with a hardy people. Much of the success of the industry established here can be attributed to that spirit. So, too, has been the establishment of scientific stature in the United States. By the same token, the fact that the country was undeveloped slowed progress initially. Early contributions to science from America were sporadic. Notable exceptions to this include Joseph Henry (1797 to 1878) who gained fame for his experiments on electromagnetic induction, Josiah Willard Gibbs (1839 to 1903) who was responsible for much of the basic work in modern chemical thermodynamics, and Jean Louis Rodolphe Agassiz (1807 to 1873) who was a leading geologist, paleontologist, and zoologist. In fact, until a few generations ago, many American scientists went abroad for their advanced education. This began to change about the beginning of the twentieth century, and the change was brought about by true pioneers. The example of William Weber Coblentz (1873 to 1962) dramatizes this.
Advances in optics, as in most branches of science, result from achievements in technology which make possible more refined observations. Although the converse is a bit rarer, advances in optical technology are also achieved by an improved understanding of the nature of light. An outstanding example of this is the invention of holography. Although it represents a form of photography, holography constitutes a distinct departure from the traditional practice of pictorial recording. It achieves this departure by being based upon a knowledge of the constitution of light. It is of interest to review the differences.
In addition to examining the obstacles man faced in his search to determine the nature of light, one naturally inquires if some pattern exists whereby we can identify the men who made significant contributions to optics. Did they do their work in their younger years? Did they generally have extensive educations? Were their lives free (or full) of outside pressures such as political entanglements, religious bigotry, or pestilence? Were they liberal or conservative in their outlook? Were they emotionally stable? Indeed, can one categorize these men who made substantial improvements in our knowledge of optics by any other attribute?
This section contains the title page, the table of contents, the preface, and an introduction.
Our comprehension of scientific achievements is often limited to those developments recorded in one’s own country or in those allied to it. That this is so is probably attributable to patriotic chauvinism and religious prejudice. For instance, we are likely to consider that the optics we practice (as well as many other cultural activities) began in Greece and came to us directly by way of Europe. Let us consider the Arabian influence on optics and dispel some of this myth.
Technology is sometimes considered to be the handmaiden of science. However one considers their association, it is a close, important, and necessary relationship. Early optical instruments were devised for utilitarian purposes, primarily to aid astronomical observations. These observations, in turn, permitted more accurate measurements of stellar positions to be made, which led to an improved understanding of the passage of light through the atmosphere. From this, it was possible to unlock the mysteries of the rainbow, the colors of the clouds and sky, and other properties of the nature of light. All this took time, of course, and required a continual improvement in optical technology.
The nature of light is so subtle that a creative investigator must couple his ingenuity with instrumentation capable of achieving high precision to observe the minute nuances attributable to light. The ancients secured significant data with instruments which might be considered crude if judged by today’s technology. From that data, however, they measured the size of the earth, erected precisely oriented monuments, deduced the height of the atmosphere, conjectured about the formation of rainbows, and studied image formation with lenses and mirrors. Nonetheless, these data were insufficient to permit a full description of what is meant by light. That required instrumentation capable of much greater measurement precision than that achieved by the sixteenth century.
Most optical instruments in use today employ glass components. How man learned to manufacture glass is conjectural. However, the origin of optical components made of glass is reasonably documented. Similarly, the impact such instruments made in scientific investigations is well known.
The efforts of the pre-Seventeenth Century scientists were largely individual undertakings. Intercourse with others was hampered by the scarcity of learned men and the slowness of the early postal systems. As scientific endeavors progressed, there was an increasing desire to discuss one’s observations with one’s peers. Gradually, men began to meet informally to discuss scientific achievements and to carry out experiments.
In the Seventeenth Century, all branches of science, including optics, showed a remarkable development. It was largely a period which science was placed on new foundations as the sterile tradition under which science had languished was destroyed. From the publication of Gilbert’s De magnete in 1600 to Newton’s Principia in 1687, the face of science changed almost beyond recognition. Scientific societies were formed, many with support from the government or with aristocratic patronage. Newton's "experimental physics" now spread from England and Holland to the rest of Europe, resulting in spectacular advances in the study of electricity, magnetism, chemistry, and geology - as well as mechanics and optics.
The paths to greatness are as numerous and diverse as the roads to Rome. This is typified by the career of Dominique Francois Jean Arago (1786 to 1853). Although one might argue that his name may be excluded from the list of greats in optics, I contend that his researches, his lectures, and above all else, his influence on other investigators, mark him as one who has had a profound leverage on our understanding of optical phenomena. Before we turn to these achievements, however, it is of interest to review his early life.
The patronage of science by ruling governments began in the days of antiquity. However, the influence of the ruling power on the development of science is particularly marked by the career of Emperor Napoleon Bonaparte. Napoleon was born in Corsica on August 15, 1769, and attended military school there. He was regarded as taciturn and morose, but since he spoke little French in this French-speaking environment, this might have been expected. He distinguished himself in mathematics, did tolerably well in history and geography, but did poorly in Latin and general literature. In 1784 he left for Paris, where his excellence in mathematics permitted him to finish his military training and receive a commission a year later instead of in the two years normally required.
In his eulogy of Joseph Fourier to the Paris Academy of Sciences, Francois Arago concluded, “My object will have been completely attained if … each of you have learned that the progress of general physics, of terrestrial physics, and of geology will daily multiply the fertile applications of the Theorie de la Chaleur, and that this work will transmit the name of Fourier down to the remotest posterity.” Although Arago thus predicted a legacy of the use of Fourier’s mathematical treatment used to describe the conduction of heat, in 1833 he could hardly envisage those benefits to be derived in optics. Today, Fourier optics and Fourier transform spectroscopy are widely practiced by scientists with little knowledge of the life of Joseph Fourier.
At the end of the Eighteenth Century, there was Still considerable uncertainty as to whether the wave theory or the corpuscular theory of light best accounted for the observed phenomena. In 1801, Thomas Young presented experimental evidence that light is subject to superposition (or interference) which suggested that “Light is probably the undulation of an elastic medium.” This hardly put the matter to rest, as we shall see. Moreover, the resulting controversy brought so much turmoil into Young’s life that he tended to avoid further research regarding the nature of light. However, his genius was not extinguished. Let us examine some of the details of his life.
The Nineteenth Century had hardly begun when Thomas Young (1773 to 1829) fanned the fire smoldering between supporters of the wave and the corpuscular theories of light. Young’s demonstrations that light can be superimposed (i.e., cause interference) gave strong evidence for the validity of the wave theory. However, within a decade Etienne Malus (1775 to 1811) discovered that light can be polarized by reflection. It was soon shown that light so polarized can also suffer interference. If light consists of longitudinal waves as Young had thought—and Christiaan Huygens (1629 to 1695) had proposed—how would it be possible for such waves to be both polarized and to interfere? Augustine Jean Fresnel recognized that light must be a transverse wave. His colleagues were slow to understand this, but Fresnel persisted with complete success.
Writing technical material in an informative manner for the layman is an art that few of us possess. It requires both a thorough understanding of science and an ability to express it in terms that are at once accurate and interesting. Many of the so-called popularizers of science today fail, in my opinion, because they sacrifice accuracy to provide an exotic impact. One of the early attempts to present science in meaningful terms for the layman (and possibly still one of the best ever) was Leonhard Euler (1707 to 1783). His Letters to a German Princess are masterpieces of scientific exposition.
William Herschel (1738 to 1822) had shown in 1800 that optical radiation extends beyond the visible red rays, but his thermometers were too insensitive to detect energy beyond that quite close to the visible spectrum. Another quarter of a century elapsed before technology advanced sufficiently for such measurements to be made. The measurements that were then made were beset with obstacles arising from political upheavals and cholera epidemics as well as scientific uncertainties. These probably would have deterred most of us, but one man of sterner stuff undertook pioneering research that opened a new vista in optical phenomenology.
During the Nineteenth Century, advances in technology, a changed economic picture, new political structures, and an evolution in the attitude toward morals affected the way science was practiced and evaluated. Since the historian of science places emphasis on scientific accomplishments, these anecdotes must reflect the underlying activity responsible for these gains in optics. A brief overview of some of the general scientific activity will thus enable us to better appreciate how optical advances were achieved during this period. The study of heat and its transformation was one of great intellectual, as well as technical and economic, importance. The principle of the conservation of energy was possibly the greatest physical discovery of the mid-nineteenth century. It showed that mechanical work, electricity, and heat are different forms of energy, and thus brought together several facets of science. The laws of thermodynamics indicated that not only the quantity of energy, but its availability, are what matter. The knowledge of thermodynamics also began to play a larger part in such fields as chemistry and biology in the nineteenth century. It was also to make its mark on optics.
Today, optical astronomy makes frequent use of satellites and rockets to enable observations to be made from above the turbulent and partially opaque atmosphere. Although some of these observations can be made with computer-controlled instruments, human observers are still frequently required. Astronauts have justifiably become modern heroes as a result of the risks and daring associated with their missions. It would be a mistake, however, to consider that risk and daring are innovations brought to optical observations by the space age. Reflect, for example, on the exploits of Samuel Pierpont Langley (1834 to 1906).
By the Mid-Nineteenth century the wave theory of light was well established. One could thus consider relationships existing in a specified spectral region. In fact, the theorem of Gustav Robert Kirchhoff (1824 to 1887) considered the fraction of energy incident in a narrow spectral band on a cavity maintained at a given temperature. He showed that the ratio of the emission to the energy absorbed in that spectral band must be a constant, and that this constant represents an intensity distribution which is a universal function. Such a function is dependent only on the temperature and wavelength, and not on the size, shape, or material of the cavity.
Although one cannot advocate wars to further the advance of science, there can be gains realized during the pressures of hostilities that are beneficial to mankind. The development of infrared sensitive detectors during World War II is a case in point. Research on photosensitive cells which was conducted by the adversaries enabled scientists in the succeeding peace to undertake investigations that have advanced astronomy, geology, medicine, agriculture, meteorology, and other scientific disciplines.
Science needs improved technology to provide instrumentation to gather data from which insights can be made to advance science. Technology, by the same token, requires advances in science before technological improvements can be scored. If this sounds like one is trying to lift oneself with one’s bootstraps, consider the events leading to the coating of the Palomar telescope mirror. To do this, we must briefly review the career of John Donovan Strong (b. January 15, 1905).
This section contains the bibliography and the index.