Advancements in Optics, 700-1449
Advancements in Optics, 700-1449
The interest in the study of the laws and phenomena of optics was one of the most popular and potentially important scientific pursuits of the Middle Ages. As such, it was an influential avenue to the development of experimental scientific philosophy. Grounded in the optical thought of the Greeks in both European and Islamic intellectual traditions, the initial stages of the advance in optical study were the translation of ancient thought, Islamic commentary on the subject about the twelfth century, and European contributions built upon Islamic influences. The most original Islamic investigator was Ibn al-Haytham (in the Latin West, Alhazen), and his influence passed to both later Islamic and European scholars. By the thirteenth century, Greek theories of vision and concepts of light, the reflection and refraction of light, application of optics with burning mirrors and lenses, and most importantly, understanding the atmospheric phenomena of the rainbow and the halo became areas of research to European thinkers, especially the English Franciscans. The physical problem of the rainbow mechanism was the major area of practical research and was finally completely explained in Dietrich of Freiberg's theory of the early fourteenth century. Other strides were made in theory and application of accurate lenses and mirrors, as well.
The study of optics, or the principles that determine the image-forming properties of reflecting surfaces (as water and mirrors) and transmitting media (water, glass, and its formation, as in lenses) in relation to light, is of ancient origin. In China, written evidence of such study (the Mo Jing or Mohist Canon) dates between 450 and 250 b.c., which parallels early Greek optical thought. In both East and West, theory focused on light sources, vision, shadows, and reflection. Vision theory rather than the study of light phenomena was the central theme of Greek optics (from the Greek optika, relating to the eye). Mathematician Euclid of Alexandria (about 300 b.c.) noted that light traveled in a straight line. Euclid may have been the origin of Greek belief and its dissemination that vision was a matter of rays being emitted by the eye (called "opseis") rather than light entering the eye to form images. Engineer Hero of Alexandria (first century a.d.) studied lines and angles of light reflection from mirrors as aspects of geometrical optics. And astronomer Claudius Ptolemy (fl. a.d. 139-161) studied theoretical and atmospheric refraction, noting the angle of refraction and incidence as unequal.
Greek optical theory, heavily influenced by Aristotle (384-322 b.c.), passed rather piecemeal with other Greek science to Europe through neo-Greek and later Roman thinkers, such as Seneca (a.d. 1-65) and Pliny the Elder (a.d. 23-79). The Latin encyclopedists, Cassiodorus (480-575) and Isidore of Seville (560-636), drew from these, providing a rather limited foundation of ancient science to the Latin Middle Ages. A much more thorough rediscovery and translating of Greek thought from Greek, Syriac, Persian, and Sanskrit sources passed into Arabic during the ninth century at Baghdad. These translations would serve to focus commentaries by Islamic thinkers, and these thinkers followed Aristotle in defining aspects of nature under the discipline of physics. The mathematical grounding of traditional optical study took on more of this physical character as investigation turned to experimental techniques.
Early Islamic thinking on the various subjects of optics included substantial thought from Yaqub al-Kindi (c. 801-870), hints of broadening of the physical subject matter of optics as in al-Farabi (d. 950), and discussion of applications, such as in Abu Sad ibn Sahl (c. 984). But Ibn al-Haytham (965-1039), Alexandrian mathematician and astronomer, was the first in the study of optics to emphasize critical deductive and inductive reasoning by way of experimentation. Among many works, he composed his comprehensive Book on Optics (c. 1027), which contained his treatment of vision and light. Ibn al-Haytham did not believe rays emanated from the eye but that light rays were received by it, and he correctly explained vision. In his theory of light, Ibn al-Haytham extended optics to the study of light propagation and the full definition of physical and geometrical optics with extensive experiments in reflection and refraction (he outlined the basic laws of refraction). Other treatises specialized in the geometry of spherical and parabolic mirrors and lenses and applications of these for causing heat and burning, the analysis of the light of the moon and stars, and the atmospheric optical phenomena of the rainbow and halo. From this initial presentation of a wider definition of optical subjects, the narrow meaning optica would be eventually replaced with the more suitable word, perspectiva.
The origin of the rainbow was a central theme of medieval optical study, particularly because Aristotle provided a detailed theory of it in his important four-part treatise on the physical earth Meteorologica. Ibn al-Haytham included experiments on the rainbow in his Optics but wrote a separate detailed work on atmospheric optics called On the Rainbow and Halo, providing further experimental depth. To Aristotle, the rainbow was a reflection phenomenon from clouds made up of uniform drops that acted as a continuum surface, something like a convex mirror. Ibn al-Haytham, who established the precedent of a laboratory for experimental optical research, decided from observing the reflection of light from plane and spherical mirrors that the rainbow was the result of reflection as from a spherical concave mirror. He simulated the rainbow colors by transmitting sunlight through glass spheres of water, which he thought simulated spherical concave mirrors and individual clouds (still acting as Aristotle's continuum, not individual drops). He did not consider his innovative spheres as representing cloud droplets, and he ignored the possibility of refraction taking part in the phenomenon, which were keys to later rainbow theory.
Ibn al-Haytham also dealt with other questions of atmospheric optics. He appears to be the first thinker to note the property of refraction of light in the atmosphere (A Question Relating to Parallax). He applied this bending of light as the reason for the distortions of celestial objects found by astronomical observers near the horizon. He seems to be the first medieval thinker to apply basic refractive theory in relation to the atmosphere as a means of determining a reasonable height of the atmosphere. Actually, his contemporary the Cordoban Moor al-Jayyani ibn Muadh (c. 989-1079) developed a more scientific method of studying atmospheric height by way of refraction. His treatise On the Dawn (also known as On Twilight and the Rising of Clouds) was mistakenly attributed to al-Haytham. Ibn Muadh noted that sunlight seen before dawn and after sunset was a refractive phenomenon. He estimated the angle of depression of the sun at dawn and sunset to be a fairly accurate deduced angle of 18°. From this he determined the height of atmospheric moisture (believed responsible for twilight), which was considered proportional to atmospheric height.
Ibn al-Haytham's level of optical comprehensiveness was not to be seen again in the Islamic world. Smaller studies and application of optics appearing in astronomical and meteorological works, particularly with the continued interest in the rainbow, characterized subsequent Islamic scholars. His near contemporary, the Persian physician Ibn Sina (Avicenna, 980-1037), devoted over 20 volumes to physical science and independently dismissed Aristotle's cloud continuum and Ibn al-Haytham's spherical mirror analogy in explaining the rainbow mechanism. Anticipating an important clue in the water drops themselves, he theorized the rainbow was the result of reflection of light from the total amalgamation of water drops released by clouds as they dissolved into rain. This idea followed from his careful observation of sunlight diffracted by water drops formed while watering a garden.
It remained for two Islamic thinkers some two hundred years later to follow in al-Haytham's optical footsteps, both representing Islamic science in the philosophical view in the later medieval period. Astronomer Quib al-Din al-Shirazi (1236-1311) inspired his student Kamal al-Din Farisi (c. 1260-1320) to delve critically into al-Haytham's optical theories. Farisi eventually and comprehensively appraised all of al-Haytham's optical works in his book Revision of Optics, in which he openly noted al-Haytham's incorrect theories. Particularly, he researched the rainbow mechanism using al-Haytham and Ibn Sina's ideas as a base to arrive at most of the correct theory with lucid conceptual physics and logical geometry superior to al-Haytham. He realized al-Haytham's spheres of water were like cloud drops, not a mass of cloud, and that the rainbow was the result of two refractions of sunlight on and in the cloud drop with one reflection, though he could not satisfactorily explain the rainbow colors. The full solution slightly antedated him and came from late medieval Latin Europe.
The fragments of Greek science that had filtered to the Latin West, providing a sketchy foundation for optical study, were finally fleshed out with the vast effort of translating Greek sources and commentaries from Arabic into Latin in the twelfth and thirteenth centuries. The core of this work was carried out under Gerard of Cremona (1114-1187) and his group of translators at Toledo. Because Islamic translations from Greek thinkers were sometimes summaries rather than true full translations, Gerard and those succeeding him, concentrated on complete translations out of Greek of Aristotle, Plato, Archimedes, Euclid, and, among others, Ibn al-Haytham's great optical treatise. Compared to Islamic familiarity with Aristotle, the presentation of his works in Europe was something of a novelty, and it spurred a new level of European scientific scrutiny, one area being in optical research.
The most important twelfth- and thirteenth-century emphasis on optics was the work of English Oxford Franciscans and their associates at the University of Paris, developing seminal divergence from Aristotelian physics. Their initial guiding light was not a Franciscan, but Robert Grosseteste (c. 1168-1253), bishop of Lincoln and a chancellor of Oxford University. Grosseteste was an early proponent of Aristotle's cause-and-effect logic, and he emphasized the importance of observation and analysis. Grosseteste was well acquainted with the physical/geometrical optics of perspectiva passed from Islamic thinkers and he experimented with the magnifying glass. He was evidently the first European investigator to conclude from his own observations that refraction of light on cloud drops would explain the rainbow but provided no detailed theory.
Following close behind came famous disciples: fellow Franciscans Roger Bacon (c. 1214-1294) and John Peckham (after 1230-1292), German Albertus Magnus (c. 1193-1280), and Polish scholar Witelo (b. c. 1230). Bacon experimented with magnification using convex lenses and was the first to suggest lenses could correct poor eyesight. In his Opus Maius (c. 1267) he conjectured that the speed of light was finite and traveled as sound did and, though short on precise theory, he believed that reflection and refraction in "numberless drops of water" (cloud drops) generated the colors of the rainbow. Peckham, who would become archbishop of Canterbury, contributed to the advance of optics by his precise simplification of the abstractions in al-Haytham's Book on Optics in his Perspectiva communis (1277-79). This work carried along contemporary emphasis on the importance of both reflection and refraction in the generation of the rainbow.
The non-English associates of the Franciscans at Paris were seminal figures in evolving the medieval science corpus. Albertus Magnus (Albert the Great, a Dominican becoming acquainted with Grossteste's ideas at the University of Paris about 1240) followed the scheme of developing formal scholastic analysis of Aristotle with commentaries on his major physical science works. He devoted much space in analyzing the Meteorologica, particularly on the rainbow, where again theoretical definition was confined to stating there was reflection and refraction of "rays of light" in "descending raindrops." Witelo (or Vitelo) studied at Paris about 1253 and became engrossed in al-Haytham's Book on Optics, which inspired his own great optical work Peri optikes or Perspectivae (between 1270 and 1278), the standard western text for some 300 years on basic optical principles. In discussing the rainbow he stressed the reflection of light on clouds and refraction through cloud drops as a necessary medium acting as "spherical lenses." Witelo fairly defined the halo as refraction (actually the more diffuse phenomenon of diffraction) of light through the sun or moon. And he developed a method of machining parabolic mirrors from iron.
Though interest in the properties of lenses and mirrors continued through the period, the resolution of the rainbow was still the fundamental optical question. Kamal al-Din Farisi had come close to the complete answer, but the more comprehensive theory of Dietrich (or Theodoric) of Freiberg (c. 1250-1311), a Teutonic member of the Order of Preachers who had studied at Paris about 1297, preceded him. Unlike traditional medieval optical literature, Dietrich's treatise On the rainbow (about 1304) was not padded with commentary on other thinkers' ideas. He, like Farisi, followed al-Haytham's lead with glass spheres filled with water as an experimental base and realized that these acted as individual cloud drops. He correctly described and depicted the primary rainbow as two refractions on and one internal reflection in a cloud drop, and the secondary rainbow (the first appearance of such in detail) as requiring an additional internal reflection (thus the inversion of the rainbow colors). He further became the first thinker to explain correctly each of the colors of the rainbow phenomenon as requiring a definite angle of incidence of sunlight, enhancing the understanding of the nature of light. He also correctly deduced that the angle/color relationship explained the circular appearance of the rainbow.
Amid a great mass of optical commentaries, Dietrich's impressive empirical presentation was but weakly echoed in the commentary of the Meteorologica of Themon Judaeus (fl. 1370) of the Paris School. The Paris School was a group of important physical theorists focusing on the causal relations in physical motion, and they did not extend their investigation to optics. And unfortunately, through the sixteenth century, Dietrich was essentially forgotten amid the printing flurry of Aristotelian commentaries of past thinkers. But within the span from Ibn al-Haytham to Dietrich, optics had been molded into a true discipline of physical science.
WILLIAM J. MCPEAK
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