OPTICS. – Originally, this term denoted that branch of physics concerned with the modalities of luminous phenomena. By "light" is meant a certain entity that, whether directly or indirectly, emanates from surrounding bodies and renders them visible to our eyes. In relation to light, bodies are distinguished as luminous or dark. Luminous bodies, or sources of light, are those from which light emanates directly; every body becomes luminous when its temperature is raised considerably (e.g., gas lamp, electric lamp, etc.). The others, being dark, are distinguished as opaque or transparent according to whether light is arrested by them or not.
The visibility of dark bodies occurs insofar as the light striking them is reflected by them to a greater or lesser extent in all directions. This is the phenomenon of the diffusion of light, which takes place when the surface of the body struck by light is rough. In the opposite case, that is, when a beam of light reaches a smooth, flat surface (e.g., a metal plate), it is reflected in a single direction, or, as it is commonly said, reflected. The direction of the reflected light beam depends on that of the incident light according to a physical law known as the law of reflection.
If the smooth surface of the body on which the light falls is not opaque but transparent, as, for example, the free surface of water, the light beam divides into two parts, one of which is reflected in the manner described above, while the other penetrates into the interior of the body, though generally following a different direction from that of the incident light beam. In such a case, it is said that the light has undergone refraction. The laws governing the phenomenon of refraction are two; experimentally demonstrated by Snell in 1615, they were later theoretically rediscovered by Descartes in 1637, so that they are generally known by the name of Descartes’ laws.
The laws of reflection and refraction, together with the two principles of the rectilinear propagation of light and the dependence of light rays, essentially encompass the fundamental conceptions acquired over many centuries of investigation into the nature of light and the modalities of its phenomena. These investigations begin with the brilliant and somewhat fanciful intuitions of the Greek thinkers from Pythagoras and Democritus to Plato and Aristotle; they continue with the Optics of Euclid (a mere treatise on the geometry of vision and reflection, linked to Pythagorean conceptions), the Catoptrics of Archimedes, the Optics of Hero of Alexandria (in which the principle of the rectilinear propagation of light and the law of reflection are enunciated), and finally with the Optics of Ptolemy (which contains the first systematic experimental studies on refraction), arriving at the Optics of Alhazen (al-Hasan), an Arab researcher who lived around the year 1000. An independent and original investigator, Alhazen gathered in his work, which nonetheless retains a distinctly geometric character, numerous experimental data, keen observations on the mechanism of vision, and a study of refraction in a sphere, from which his commentator Kamāl al-Dīn al-Fārisī deduced the explanation of the rainbow, among other things.
That light propagates in a straight line is a matter of everyday observation; this occurs even over distances far greater than terrestrial ones, as the existence of eclipses attests.
The principle of the rectilinear propagation of light does not hold rigorously, but only as a first approximation. Light emitted from a point source, passing through an aperture in an opaque screen, produces an image exactly aligned with it: this is proof of the rectilinear propagation of light. But if one gradually reduces the size of the aperture, it is seen that, when the dimensions become very small (less than a millimeter), the image of the aperture widens and appears surrounded by rings, alternately dark and bright (the latter iridescent if the light from the source is white). The image is thus no longer aligned with the aperture: this means that the light coming from it does not propagate in a straight line but scatters in all directions. Whenever light from small sources passes through very narrow openings, regardless of their shape, this phenomenon occurs, known as the diffraction of light. It also appears when small obstacles (wires, etc.) or very sharp edges obstruct the path of light.
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OTTAWA, ARCHDIOCESE OF – Interior of the Cathedral (19th century).
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It was impressed upon to ensure that both the English and French peoples had the right to instruction in their respective languages that the government of the United Canadas decided (4 July 1866) to grant it the civil status of a university. Leo XIII conferred upon it (5 Feb. 1889) the status of a university, and the confirmation of its government constitution by the apostolic constitution *Deus Scientiarum Dominus* was approved again by Pius XI (15 Nov. 1934), constituting the first case of its kind on the entire American continent. The civil charter was also updated and approved by the Parliament of Ontario on 18 Apr. 1933.
From its very beginnings, Thomist philosophy and the sciences were cultivated in a particularly distinctive manner; however, the specifically academic development of the University truly began in 1923. At that time, a bilingual normal school (Institute of Pedagogy) was established, which four years later became the official school of the government of Ontario. In 1929, the already existing School for Nurses at the General Hospital of the Grey Nuns of O. was affiliated with the University and, in 1940, became a University School. In the same year, 1929, a pre-medicine course was established, which was transformed in 1945 into a Faculty of Medicine with its own university hospital, generously provided by the Grey Nuns. The School of Sacred Music, founded in 1932, became, three years later, the present School of Music and Declamation. In 1934, an Institute of Philosophy was established to accommodate young laypeople desirous of higher education, and in the summer of 1936, the St. Paul University Seminary was opened for ecclesiastical students of the Faculties of Theology, Canon Law, and Philosophy. Notable is the Library of this latter institution, which ranks among the richest in North America.
A special mention is deserved by the Faculty of Arts, the original nucleus of the University of O., whose progressive development marks the rhythm of the evolution of the entire institution. The Faculty of Science was founded in 1935; that of Commerce in 1937; and the Institute of Geography in 1951. Summer courses began in 1935, correspondence courses in 1936, evening courses in 1937, and a graduate school was instituted in 1949. The School for Librarians, the School of Political and Social Sciences, the Institute of Experimental Psychology with its Education Section, the School of Catholic Action, the School of Applied Sciences (Engineering), the Institute of European Studies, the International Institute, the Institute of Physical Education, the Faculty of Law, and the Institute of Missiology complete the series of schools and faculties. The Catholic Center and the Social Center disseminate Catholic and social doctrine throughout the country and the United States. The scientific activity of the University of O. is documented by the *Revue de l'Université* and the *Editions de l'Université d'O.*
From daily observation it can also be deduced that two or more beams of light can cross without the path or intensity of each appearing to be affected by the other. This is the principle of the independence of light paths, which is open to criticism similar to that directed against the principle of the rectilinear propagation of light. It is indeed possible to establish particular conditions that give rise to the phenomenon of light interference. As early as the 17th century, Francesco M. Grimaldi (to whom belongs the credit for the discovery of diffraction) collected on a screen the cones of light that emanated from two small holes made close together in a window shutter and observed the appearance of alternating light and dark bands. However, it was only at the beginning of the 19th century, after interference phenomena had been reproduced and observed in numerous and varied experiments (among which Newton’s rings remained famous) without a plausible explanation being given, that Thomas Young carried out the interference experiment envisaged by Fr. Grimaldi in a precise manner, at the same time providing the correct interpretation. The modification introduced by Young to Grimaldi’s experiment consisted in illuminating the two small holes not with light coming directly from the sun, but with light that had first passed through a single small hole, so as to experiment with two light beams emitted from a single point source, the hole illuminated by the sun. Under these conditions, on the screen that collects the two light beams, and precisely in the zone where they overlap (having been widened by diffraction), an alternation of dark and bright bands appears (the latter iridescent), the so-called interference fringes: the principle of the independence of light rays is no longer satisfied.
A fundamental problem of optics concerns the nature of light. The various hypotheses formulated since antiquity about the nature of light converged into two main currents. One of them, whose origin can be traced back to the atomistic conceptions of Democritus, identified light with an effluence of particularly tenuous atoms that, emitted by luminous bodies in straight-line trajectories, reach the eye and produce vision. Among the supporters of this “corpuscular” theory are Gassendi, Descartes, and above all Newton.
More than a century earlier, taking up the Aristotelian conception, which attributed vision to disturbances produced by the colour of a body or by fire or other agents in an indeterminate medium filling all space, Leonardo da Vinci had wondered whether the propagation of light might not occur in a manner analogous to the propagation of disturbances excited in liquids, as, for example, when a stone falls into still water, that is, by waves; so that, as he himself admirably expressed it with regard to water, “the motion of the impression is accompanied only by the impetus and not by the motion of the water itself.” The true initiator of the wave theory, however, was Christiaan Huygens: according to the great Dutch physicist, light is nothing other than a movement “by waves and spherical surfaces” of a certain matter pervading all space, to which he gave the name *ether*, just as sound consists in an analogous movement of the air. Such waves move “with a successive motion” without transporting the “ethereal matter”: from each individual ethereal particle the motion is communicated to contiguous particles in such a way that each becomes the centre of a secondary wave whose propagation is independent of that of the others.
The envelope of all the secondary waves generated by a single wave constitutes the wave that follows II. This is the content of Huygens’ principle (1678), from which he deduced in a simple and immediate way the interpretation of the phenomena of reflection and refraction, without realising that he could just as easily have explained Grimaldi’s diffraction experiments with II.
Nevertheless, more than a century was to pass before the substantial clarification brought by this principle was fully understood in its importance.
The corpuscular theory had as its supporter Isaac Newton, whose authority was such that the wave hypothesis was practically abandoned at the beginning of the 18th century. From this time onward, optical research was entirely based on Newtonian conceptions and was practically directed towards overcoming the enormous obstacles encountered in attempting to interpret on this basis the phenomena of diffraction, interference, and dispersion. For while the corpuscular theory can provide a more or less valid and persuasive interpretation of the experimental laws of reflection and refraction (admitting, as, for example, Gassendi did, that reflection occurs by a mechanism analogous to the rebound of an elastic ball against a wall, and that refraction is to be attributed to the oblique impact of light atoms against the refracting medium), unless recourse is had to particularly cumbersome and convoluted mechanisms, which could almost be defined as absurd, the same theory fails to explain deviations from the principle of rectilinear propagation and from the principle of the independence of light rays, or the phenomenon of dispersion.
The latter phenomenon was first observed by Newton, who, directing a beam of sunlight from a narrow slit onto a glass prism, observed on the screen receiving the light at the prism’s exit not the image of the slit but a luminous band or spectrum, in which the colours of the rainbow succeeded one another without interruption. Newton was also able to achieve the synthesis of the colours in sunlight and understood that the latter is formed by the mixture of lights of different colours. The dispersion, that is, the decomposition of light into its individual components, is effected by the prism by a mechanism of which the English physicist, misled by his corpuscular conceptions of light, was unable to grasp.
It was only at the beginning of the 19th century that the wave theory was revived by Young, who, having supposed that ethereal particles oscillate about their equilibrium positions in the direction of propagation of light, succeeded in interpreting the experiment he had devised, following Grimaldi’s, as a phenomenon of reinforcement and cancellation of wave motion, and was also able to explain the chromatic phenomena in thin films and the colouration of striated surfaces.
The results of Young’s researches were not received favourably by his contemporaries. Meanwhile, a great ferment of experimental investigations into light was spreading everywhere: Herschel and Ritter observed that the luminous spectrum extended beyond the red and, respectively, beyond the violet; Young himself realised that radiant heat differed from light only in having a greater wavelength; Malus finally demonstrated experimentally that light, after undergoing reflection or refraction, or after passing through certain particular crystals, acquires certain characteristics that vary with the azimuth relative to the direction of propagation. The latter phenomenon was called by Malus himself—who attempted to explain it on the basis of corpuscular conceptions—polarisation of light.
A rich harvest of experimental data, together with the considerations of Young, was gathered by Augustin Fresnel, who between 1814 and 1816 developed a mathematical theory that came to form, so to speak, the framework of the wave hypothesis, while at the same time devising and carrying out (alone or in collaboration with F. Arago) a series of experiments to confirm and consolidate this theory. Taking as his starting point Huyghens’ principle and confirming Young’s interference concepts, he succeeded in explaining the phenomena of diffraction, dispersion, and polarization in the form in which they are still interpreted today.
The essential conceptions regarding the nature of light and its propagation, which constitute the main result of Fresnel’s scientific work, may be summarized as follows: a) the mechanism of light propagation consists in an undulatory motion of an elastic medium, the ether, which occupies all space; this motion takes place according to the modalities established by Huyghens’ principle. It follows that the undulatory character of light propagation is not observed unless the light encounters on its path obstacles whose dimensions are comparable to its wavelength (diffraction). b) The wavelength of visible light radiations lies between 4 and 8 x 10^-5 cm. The difference in wavelength of a radiation appears to our visual sense as a difference in color, so that each color is characterized by a well-determined value of the wavelength. Sunlight and, in general, all those light radiations commonly called white are the result of the superimposition of radiations of all wavelengths, i.e., of all colors. The dispersion of light through a prism separates the different wavelengths, i.e., the different colors. c) The motion of the ether particles at a point is the resultant of the motions produced by the action of individual light sources; from this follows the interpretation of interference phenomena according to Young’s conceptions. d) The oscillations of the ether particles occur perpendicularly to the direction of propagation of the light.
This last assertion, in contrast to Young’s hypothesis, according to which light oscillations were thought to occur along the direction of propagation, is the result of Fresnel’s studies on the phenomenon of polarization, for which he provided a simple and convincing interpretation.
Fresnel’s work gave a new impetus to optical research and, on the other hand, stimulated a flourishing series of investigations into elasticity, since the hypotheses concerning the mechanism of light propagation on which Fresnel’s theory was based posed a series of new problems in the field of elasticity. The main achievements, reached around 1850 through the work of a great number of distinguished scholars (Poisson, Cauchy, Green, and Lamé), may be summarized in the following statement, which was to have a great influence on subsequent research: the ether behaves, with respect to light propagation, like a perfectly rigid, incompressible elastic solid; in it, light propagation occurs through transverse waves.
The enormous difficulties involved in reconciling the concept of a rigid solid ether with the freedom of movement of bodies, especially celestial ones, are easily imaginable. In particular, it was deduced that, as a consequence of the Earth’s rotation, the Earth should be struck by an ether wind and at the same time drag the ether along with II.
Ancient thinkers, led by everyday observation, believed that the propagation of light was instantaneous; but already Hero rejected the hypothesis of instantaneousness, affirming instead that light propagation must be incomparably rapid. Galileo addressed this problem; he intuited that the speed of light must be finite, and to prove this he devised some experiments (1638) without, however, succeeding in his aim because of the inadequacy of experimental means. The attempt was renewed in 1663 by the Accademia del Cimento, without any result. In 1676 the Danish astronomer Römer noted a periodic irregularity in the appearance of the eclipses of Jupiter’s first satellite; Huyghens, for his part, who was also convinced that light propagation occurred at a finite speed, attributed the periodic advances and delays discovered by Römer to variations in the time taken by light to travel the Earth–Jupiter distance, as a result of the different positions occupied by Jupiter relative to the Earth.
On the basis of these considerations, Huyghens calculated that the speed of light must be around 212,000 km/sec.
About two centuries after the Accademia del Cimento’s attempt, i.e., in 1849, Fizeau devised and successfully carried out an experiment for the terrestrial measurement of the speed of light. The following year, with a more elegant experimental device, Foucault demonstrated that the speed decreases in more refractive bodies; and thus he definitively disproved the corpuscular theory. Between 1840 and 1850, M. Faraday, perhaps inspired by the new theory of light propagation, abandoned the conception then universally accepted, which attributed electrical and magnetic phenomena to action at a distance between electric charges (and, respectively, between magnetic poles) and suggested a new scheme according to which the reciprocal actions between electric charges or between magnetic poles are transmitted by the intervening medium. James C. Maxwell based his electromagnetic theory on Faraday’s hypothesis, a remarkable synthesis in which electrical phenomena are interpreted as elastic actions of the medium intervening between electric charges or magnetic poles. From the four celebrated equations by means of which Maxwell expressed the properties of the perturbation, or, as it is sometimes called, of the electromagnetic field, are derived the propagation equations of an electromagnetic field in motion in a homogeneous medium that is the seat neither of free electric charges nor of conduction currents.
The formal identity existing between these equations and the propagation equations of elastic waves proves that the propagation of electromagnetic actions occurs through waves and is not instantaneous, but possesses a finite speed.
These considerations led Maxwell to recognize in light waves a particular case of electromagnetic waves: the main consequence of such a supposition is that the speed of light should be equal to that of electromagnetic waves. Experience confirmed this hypothesis. The Maxwellian theory of electromagnetism, refined by Heaviside, received its definitive seal from the discovery of Hertzian waves (1888). In the following years, Hertz himself, A. Righi, and others succeeded in establishing the existence of Hertzian waves, reproducing with them the main phenomena already observed for light (reflection, refraction, polarization, etc.) in complete confirmation of the Maxwellian hypothesis according to which it could be foreseen that electromagnetic waves, generated by means of oscillators, differ from light waves only in wavelength, which varies from a few thousand meters to a few centimeters.
In 1912, Laue, using the periodic structures of crystallized matter as diffraction gratings, succeeded in demonstrating that even with the radiation discovered about twenty years earlier by Röntgen—the so-called X-rays—experiences of diffraction and interference could be obtained. Further research by Bragg, Siegbahn, Larson, and others confirmed that these radiations, which are of an electromagnetic nature and differ from light solely in the brevity of their wavelength (less than 10⁻⁸ cm), also exhibit such phenomena. Around the same period, it was also demonstrated that radioactive elements emit X-rays of even shorter wavelength (less than 10⁻¹¹ cm), to which the name gamma rays (γ) was given.
Thus, light waves definitively came to be recognized as part of a much broader category: electromagnetic waves, whose wavelengths range from Hertzian waves to gamma rays across a vast spectrum spanning from several thousand meters to 10⁻¹¹ cm. Within this spectrum, the zone of the "visible," that is, light as initially defined, occupies a narrow interval between 4 and 8 × 10⁻⁵ cm. It has been observed how the electromagnetic theory was expressed by Maxwell in terms derived from the theory of elasticity. The analogy is purely formal, yet neither Maxwell nor the scientists who later addressed these problems realized this. Consequently, for a long time, it was assumed that the propagation of electromagnetic waves occurs through the luminiferous aether. In reality, this assumption is not a necessary support for electromagnetic theory, since the concept of the aether is essential only for problems in the electrodynamics of moving bodies, as it would constitute a privileged reference system (v. RELATIVITY).
When the Michelson experiment demonstrated the nonexistence of such a system, the independence of Maxwell’s theory from any elastic model was finally recognized. Electromagnetic phenomena, including optical ones, were then framed within a logical scheme based exclusively on the concepts of electric and magnetic fields, without recourse to the hypothesis of the aether as a propagation medium. Thus, one of the greatest obstacles that had previously been raised against the affirmation of the wave theory in the form expressed by Fresnel was removed. Within this framework, light can be considered a particular action at a distance between the light source and the illuminated body, dependent on both, transmitted by no hypothetical medium and delayed in time. In other words, it can also be said that "light is a transfer of energy delayed in time."
The fact remains that experimental observation reveals only the existence of a light source, an illuminated body, and a delayed action at a distance involving an energy transfer, also delayed in time, without being able to ascertain anything about what occurs along this transfer. The localization, or rather the "visualization," of light can only be achieved through absorption processes by atoms, which fundamentally alter the phenomenon.
In other words, an experiment that would allow the detection of the passage of a photon or a wave without disturbing its path is conceptually impossible. According to Heisenberg’s principle, it must therefore be admitted that the concept of "propagation" is devoid of any meaning; just as the theory of relativity had taught us to dispense with the aether, this principle requires us to abandon the definition of a photon’s trajectory, since it cannot be detected by any experiment. It will therefore be permissible to speak of photons, provided that, while renouncing the tracking of individual photons in their motion, we limit ourselves to determining the number of photons received by the retina, a screen, or a photographic plate. The only task that theory is capable of fulfilling is to teach how to calculate this number based on the position of the screen, the source, and all the optical instruments required for the experiment. These recent considerations offer an interpretation of the electromagnetic theory of light and explain the inability to account for the photoelectric effect and other phenomena.
From a rigorous revision of the principles on which the quantum theory and the electromagnetic theory of light were founded, there has emerged, through the abandonment of all superfluous conceptions, a resolution of the conflict between the two theories, which thus come to form a unified vision.
The possibility, indeed the necessity, of resorting to either the quantum or the electromagnetic conception, depending on the type of phenomena examined, can be further clarified by the crisis that, in those same years, affected—in a sense inversely—the conceptions regarding the nature of matter. At the end of the last century, while the wave nature of light was definitively confirmed, no one doubted the discontinuous character of matter (molecules, atoms, etc.). However, as we have seen, in the following years the corpuscular conception of light increasingly gained ground. Through a critical process analogous to that followed in investigations into the nature of light, L. De Broglie arrived at the admission of the possible wave nature of matter (1924). Three years later, De Broglie’s hypothesis, developed and completed by Schrödinger, Heisenberg, Born, and others, received confirmation in the experiments of Davisson and Germer, who, following the methods adopted by Laue and Bragg for the study of X-rays, succeeded in performing a diffraction experiment with a beam of electrons. Thus arose electron optics, which has provided modern technology with the electron microscope and other valuable instruments.
The experiment of Davisson and Germer was repeated with molecular beams in 1931 by Estermann, Frisch, and Stern. Thus, the possibility for matter, like radiation, to exhibit both wave and particle aspects was definitively established.