OPTICS. — This name originally denoted that branch of physics concerned with the modalities of luminous phenomena. Light is understood as a quid which, coming—directly or otherwise—from the surrounding bodies, renders them visible to our eyes. In relation to light, bodies are distinguished as luminous and dark. Luminous bodies, or sources of light, are those from which light emanates directly: every body becomes luminous when its temperature is raised considerably (gas lamp, electric lamp, etc.). The others, the dark bodies, are distinguished as opaque or transparent according as the light is stopped by them or not.
The vision of dark bodies takes place insofar as the light striking them is sent back by them to a lesser or greater extent in all directions. This is the phenomenon of the diffusion of light, which occurs when the surface of the body struck by the light is rough. In the opposite case, that is, when a beam of light reaches a flat and polished surface (e.g., a metal plate), it is sent back in a single direction or, as it is usually said, reflected. The direction of the reflected beam of light depends on that of the incident light according to a physical law known as the law of reflection.
If the polished surface of the body upon which the light falls is not opaque but transparent, as, for example, the free surface of water, the luminous beam divides into two parts, one of which is reflected in the manner mentioned above, while the other penetrates into the body, though generally following a direction different from that of the incident light beam. In this case light is said to have undergone refraction. There are two laws governing the phenomenon of refraction; demonstrated experimentally by Snell in 1615, they were theoretically rediscovered in 1637 by Descartes, and hence are generally known as the Cartesian laws.
The laws of reflection and refraction, together with the two principles of the rectilinear propagation of light and of the independence of luminous rays, substantially comprise the fundamental conceptions acquired during the many centuries of investigation into the nature of light and the modalities of the phenomena pertaining to II. 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, connected with Pythagorean conceptions), the Catoptrics of Archimedes, the Optics of Heron 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 of refraction), and reach the Optics of Alhazen (al-Hasan), an Arab researcher who lived around the year 1000. An enquiring, independent, and original spirit, Alhazen collected in his work, which nevertheless retains a distinctly geometrical character, numerous experimental data, acute observations on the mechanism of vision, a study of refraction in a sphere, from which his commentator Kāmil ad-dīn al-Fārīsī deduced an interpretation of the rainbow, etc.
That light propagates in a straight line is a fact of daily observation; this also occurs over distances far greater than terrestrial ones: the existence of eclipses bears witness to II.
The principle of the rectilinear propagation of light does not hold rigorously, but only as a first approximation. Light emitted by a point source and passing through a hole made in an opaque screen produces an image exactly aligned with them: this is evidence of the rectilinear propagation of light. But if one is able gradually to reduce the dimensions of the hole, one observes that, when they become very small (less than a millimetre), the image of the hole widens and appears surrounded by alternating dark and light rings (the latter iridescent if the light emitted by the source is white). The image is therefore no longer aligned with the small hole; this means that the light coming from it does not propagate in a straight line but spreads in every direction. Whenever light emitted by sources of small dimensions passes through very narrow openings, whatever their shape, this phenomenon appears; it is known as the diffraction of light. It also appears when obstacles of slight dimensions (wires, etc.) are interposed in the path of the light, or at the edges of very sharp obstacles.
Daily observation also shows that two or more beams of light can cross without the path or intensity of any one of them appearing to be influenced by the other. This constitutes the principle of the independence of luminous paths, which is open to criticism analogous to that directed against the principle of the rectilinear propagation of light. It is in fact possible to establish particular conditions that give rise to the phenomenon of luminous interference. As early as the seventeenth century, Francesco M. Grimaldi (to whom the discovery of diffraction is due), collecting on a screen the cones of light issuing from two nearby holes made in a window shutter, is said to have observed the appearance of alternating bands of light and shadow. But only at the beginning of the nineteenth century, after interference phenomena had been reproduced and observed in numerous and diverse experiments (among which Newton’s rings have remained famous) without any plausible explanation having been offered for them, did Thomas Young accurately perform the interference experiment intuited by Fr. Grimaldi, while at the same time providing its correct interpretation. Young’s modification of Grimaldi’s experiment consisted in illuminating the two holes with light not coming directly from the sun, but first passed through an initial aperture, so as to experiment with two luminous beams emitted by a single point source—the hole illuminated by the sun. Under these conditions, on the screen receiving the two luminous beams, and precisely in the area where they, widened by diffraction, overlap, there appears an alternation of dark and bright bands (the latter iridescent), the so-called interference fringes: the principle of the independence of luminous rays is no longer satisfied.
The fundamental problem of optics concerns the nature of light. The various hypotheses formulated since antiquity concerning the nature of light converged into two principal currents. One of them, whose origin is to be traced to the atomistic conceptions of Democritus, identified light with an effluvium of particularly fine atoms which, emitted by luminous bodies along rectilinear trajectories, reach the eye and produce vision. Among the advocates of this “corpuscular” theory, Gassendi, Descartes, and above all Newton should be remembered.
Already 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 undetermined medium filling all spaces, Leonardo da Vinci had asked himself whether the propagation of light might not occur analogously to the way in which disturbances excited in liquids propagate—for example, when a stone falls into still water—that is, by waves; so that, as he himself expressed it admirably with regard to water, «the motion of the impression is accompanied only by impetus and not by the motion of the water itself». The true initiator of the wave theory was, however, Christian Huyghens: according to the great Dutch physicist, light is nothing other than a movement «by waves and spherical surfaces» of a certain matter permeating all space, to which he gives the name of ether, just as sound consists in an analogous movement of air. These waves move «with a successive movement» without transport of the «ethereal matter»: from each individual ethereal particle the movement is communicated to the contiguous particles, so that each of them 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 one and the same wave constitutes the wave subsequent to II. This is the substance of Huyghens’s principle (1658), from which he deduced simply and immediately an interpretation of the phenomena of reflection and refraction, without realizing that by means of it he could have explained with equal ease the diffraction experiments carried out by Grimaldi.
Nevertheless, more than a century was to pass before the substantial clarification provided by this principle was understood in all its importance.
The corpuscular theory had Isaac Newton as its supporter, and his authority was such that the wave hypothesis was practically abandoned at the beginning of the eighteenth century. From that time onward, research in o. was entirely founded on Newtonian conceptions and was directed in practice toward overcoming the enormous obstacles encountered in attempting to interpret, on such bases, the phenomena of diffraction, interference, and dispersion. For although the corpuscular theory makes it possible to deduce a more or less valid and persuasive interpretation of the experimental laws of reflection and refraction—assuming, for example, as Gassendi did, that reflection occurs through a mechanism analogous to the rebound of an elastic ball against a wall, and that refraction is attributable to the oblique impact of light-atoms against the refracting medium—without resorting to particularly elaborate and cumbersome mechanisms, which might almost be called absurd, the same theory cannot explain deviations from the principle of rectilinear propagation and from the principle of the independence of light rays, nor the phenomenon of dispersion.
This latter phenomenon was first observed by Newton, who, directing a small beam of sunlight from a narrow slit onto a glass prism, observed on the screen collecting the light emerging from the prism not the image of the slit, but rather a luminous strip or spectrum, in which the colours of the rainbow followed one another without interruption. Newton also succeeded, conversely, in achieving the synthesis of colours in sunlight and understood that the latter is formed by the mixture of differently coloured lights. Dispersion, that is, the decomposition of light into its individual components, is produced by the prism through a mechanism which the English physicist, misled by his conceptions concerning the corpuscular nature of light, was unable to understand.
Only at the beginning of the nineteenth century was the wave theory taken up again by Young, who, having supposed that ethereal particles oscillate about their position of rest in the direction of light propagation, succeeded in interpreting the experiment he devised on the model of Grimaldi’s as a phenomenon of reinforcement and cancellation of wave motion, and also succeeded in explaining chromatic phenomena in thin films and the coloration of striated surfaces.
The result of Young’s research was not favourably received by his contemporaries. Meanwhile, a great fervour 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 recognized that radiant heat differed from light only in having a greater wavelength; finally, Malus demonstrated experimentally that light, after undergoing reflection or refraction, or after passing through certain particular crystals, acquires characteristics that vary with the azimuth relative to the direction of propagation. This latter phenomenon was called by Malus himself, who attempted to explain it on the basis of corpuscular conceptions, the polarization of light.
Such a rich harvest of experimental data, together with Young’s considerations, 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 confirming and consolidating that theory. Taking his starting point from Huyghens’s principle and confirming Young’s interference conceptions, he arrived at an explanation of the phenomena of diffraction, dispersion, and polarization in the form in which they are still interpreted today.
The essential conceptions concerning the nature of light and its propagation, which constitute the principal result of Fresnel’s scientific work, may be summarized as follows: a) the mechanism of light propagation consists in the wave motion of an elastic medium, the ether, which occupies all space: this movement occurs according to the modalities established by Huyghens’s principle. It follows that the wave character of luminous propagation becomes perceptible only when light encounters obstacles in its path whose dimensions are comparable with its wavelength (diffraction). b) The wavelength of visible light radiation lies between 4 and 8 x 10⁻³ cm. The difference in wavelength of a radiation appears to our visual sense as a difference in colour, so that each colour is characterized by a specific value of the wavelength. Sunlight and, in general, all those light radiations commonly called white, are the result of the superposition of radiations of all wavelengths, that is, of all colours. The dispersion of light produced by the prism is due to the fact that the degree of refraction of a monochromatic light wave depends on its wavelength, that is, on its colour. c) The movement of the ethereal particles at a given point is the resultant of the movements that would be produced there by the action of the individual luminous sources: from this follows the interpretation of interference phenomena according to Young’s conceptions. d) The oscillations of the ethereal particles occur perpendicularly to the direction of light propagation.
This latter assertion, in opposition to Young’s hypothesis, according to which luminous oscillations occurred along the direction of propagation, is the result of the studies conducted by Fresnel on the phenomenon of polarization, to which he provided a simple and convincing interpretation.
Fresnel’s work gave a fresh impetus to research in o. and, on the other hand, prompted a flourishing of investigations into elasticity, since the hypotheses concerning the mechanism of the propagation of light on which Fresnel’s theory was founded posed a series of new problems in the field of elasticity. The principal achievements, attained around 1805 through the work of a great number of distinguished scholars (Poisson, Cauchy, Green, and Lamé), may be summed up in the following statement, which would carry enormous weight in subsequent research:
the ether behaves, with regard to the propagation of light, like a perfectly elastic and incompressible rigid solid; in it, the propagation of light takes place through transverse waves.
The enormous difficulties encountered in attempting to reconcile the concept of a rigid solid ether with the freedom of movement of bodies, especially celestial bodies, are readily imaginable. In particular, it was inferred that, as a consequence of the Earth’s rotational motion, the earth should be swept by an “ether wind” and at the same time drag the ether itself along with II.
Everyday observation led the earliest thinkers to regard the propagation of light as instantaneous; but Heron had already rejected the hypothesis of instantaneous propagation, asserting instead that the propagation of light must be incomparably rapid. Galileo returned to this problem; he intuited that the speed of light must be finite, and, as proof of this, devised several experiments (1638), although he did not succeed because of the inadequacy of the experimental means available. The attempt was renewed in 1663 by the Accademia del Cimento, without any result. In 1676 the Danish astronomer Römer reported a periodic irregularity in the appearance of the eclipses of the first Medicean satellite: Huyghens, who for his part was also convinced that the propagation of light occurred at a finite speed, attributed the periodic advances and delays discovered by Römer to variations in the time taken by light to traverse the Earth–Jupiter distance, as a consequence of the different positions occupied by Jupiter with respect to the earth.
On the basis of these considerations, Huyghens calculated that the speed of light must be approximately 212,000 km/sec.
About two centuries after the attempt by the Accademia del Cimento, that is, in 1849, Pisseu devised and successfully carried out an experiment to measure the speed of light terrestrially. The following year, using a more elegant experimental apparatus, Foucault demonstrated that the speed decreases in more highly refracting bodies; he thus definitively brought down the corpuscular theory. Between 1840 and 1850, M. Faraday, perhaps inspired by the new theory of the propagation of light, abandoned the then universally accepted conception, which attributed electrical and magnetic phenomena to action at a distance between electric charges (and, respectively, between magnetic poles), and proposed a new model according to which the reciprocal actions between electric charges or between magnetic poles are transmitted by the intervening medium. James C. Maxwell based his theory of electromagnetism on Faraday’s hypothesis, producing 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 usually called, the electromagnetic field, one derives the equations governing the propagation of an electromagnetic field moving through a homogeneous medium that contains neither free electric charges nor conduction currents.
The formal identity between these equations and the equations governing the propagation of elastic waves proves that the propagation of electromagnetic actions takes place 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 principal consequence of such an assumption is that the speed of light ought to be equal to that of electromagnetic waves. Experiment confirmed this hypothesis. Maxwell’s theory of electromagnetism, perfected by Heaveside, received its definitive seal with the discovery of Hertzian waves (1888). In subsequent years, Hertz himself, A. Righi, and others succeeded in establishing an o. of Hertzian waves, reproducing with them the principal phenomena already observed for light (reflection, refraction, polarization, etc.), in complete confirmation of the Maxwellian hypothesis, on the basis of which it could be predicted that electromagnetic waves generated by means of oscillators differ from luminous waves only in their wavelength, which varies from several thousand m to a few cm.
In 1912 Laune, using the periodic structures of crystallized matter as diffraction gratings, succeeded in proving that diffraction and interference experiments could also be obtained with the radiation discovered approximately twenty years earlier by Röntgen, the so-called X-rays. Further research by Bragg, Siegbahn, Larson, and others confirmed that these radiations, which are electromagnetic in nature, differ from light solely in the shortness of their wavelength (less than 10⁻⁸ cm). At about the same time, it was finally proved that radioactive elements emit X-rays with an even shorter wavelength (less than 10⁻¹¹ cm), which were given the name γ-rays.
Thus light waves definitively became part of a much broader category, that of electromagnetic waves, whose wavelengths extend from those of Hertzian waves to γ-rays over a vast scale ranging from several thousand m to 10⁻¹¹ cm. Within it, the “visible” region, that is, light, according to the definition given at the outset, is represented by a narrow interval between 4 and 8 × 10⁻³ cm. We have seen how the electromagnetic theory was expressed by Maxwell in terms derived from the theory of elasticity. The analogy is purely formal, but neither Maxwell nor the scientists who subsequently concerned themselves with these problems realized this. Consequently, it continued to be assumed for a long time that the propagation of electromagnetic waves took place through the cosmic ether. In reality, this assertion is not a necessary support for electromagnetic theory, since the concept of the ether is essential only for problems concerning the electrodynamics of moving bodies, insofar as it would constitute a privileged frame of reference (v. RELATIVISMO).
When Michelson’s experiment had demonstrated the nonexistence of such a system, the independence of Maxwellian theory from every elastic model finally became apparent. Electromagnetic phenomena, including optical phenomena, were then incorporated into a logical framework founded exclusively on the concepts of the electric and magnetic fields, without any need to invoke the hypothesis of the ether as a medium of propagation. Thus one of the greatest obstacles that, as previously seen, had been raised against the acceptance of the wave theory in the form in which it had been expressed by Fresnel fell away. Within this framework, light may be regarded as a particular action at a distance between the luminous source and the illuminated body, dependent on both, not transmitted by any hypothetical medium, and delayed in time: in other words, one may also say that “light is a transfer of energy delayed in time.”
The fact is that experimental observation establishes only the existence of a luminous source, an illuminated body, and a distance action delayed in time, with which an energy transfer is associated, itself likewise delayed in time, without making it possible to ascertain anything of what occurs during that transfer. The localization, or rather the “visualization,” of light can be achieved only through processes of absorption by atoms, which substantially alter the phenomenon.
In other words, an experience that would permit the detection of the passage of a photon or a wave without disturbing its path is conceptually impossible. According to Heisenberg’s principle, one must therefore admit that the concept of “propagation” is devoid of any meaning; just as the theory of relativity had taught us to dispense with the ether, this principle requires us to abandon the definition of a photon’s trajectory, since it cannot be revealed by any experiment. It will therefore be permissible to speak of photons, provided that, instead of attempting to follow each individual photon in its motion, we limit ourselves to determining the number of photons received by the retina, a screen, or a photographic plate. The sole task that theory is capable of fulfilling is to teach us how to calculate this number on the basis of the positions of the screen and the source, and of all the optical instruments required for carrying out the experiment. These recent considerations offer an interpretation of the electromagnetic theory of light and explain its inability to account for the photoelectric effect and for other phenomena as well.
Through the rigorous revision of the principles on which quantum theory and the electromagnetic theory of light were founded, and by abandoning every superfluous conception, the opposition existing between the two theories has been overcome; they thus come together in a single synthetic vision.
The possibility, indeed the necessity, of resorting to the two conceptions—quantum or electromagnetic—according to the type of phenomena under examination, can be further clarified by the crisis that, in those same years, affected, so to speak in the opposite direction, conceptions concerning the nature of matter. At the end of the last century, while the wave nature of light was definitively confirmed, no one any longer doubted the discontinuous character of matter (molecules, atoms, etc.). It has nevertheless been seen how, in the following years, the corpuscular conception of light became increasingly established. Through a critical procedure analogous to that followed in investigations into the nature of light, L. De Broglie arrived at the admission of the possibility of a wave character of matter (1924). Three years later, De Broglie’s hypothesis, subsequently developed and completed by Schrödinger, Heisenberg, Born, and others, found 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 carrying out a diffraction experiment with a narrow beam of electrons. Thus arose electron optics, which 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. The possibility was thus definitively established for matter, as for radiation, to manifest itself under two aspects, wave-like and corpuscular.