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Optics

OPTICS

OPTICS. The development of optics between 1450 and 1789 can be conveniently divided into two phases bridged by the optical work of Johannes Kepler (15761630) and distinguished by a radical change in analytic focus. During the first phase, that focus was primarily on sight, not light. During the second, it shifted completely from sight to light. Reflecting this shift, the following essay consists of three sections, the first dealing with pre-Keplerian optics, the second with the Keplerian transition, and the third with post-Keplerian developments.

PRE-KEPLERIAN OPTICS

By 1450 two ostensibly contradictory models of sight were available to European thinkers. The first and simpler of the two harks back to the visual-ray theory of Euclid (fl. c. 300 b.c.e.). Brought to maturity by Ptolemy (c. 170 C.E.), this theory assumes that a constant stream of visual flux emanates from the center of the eye through the pupil to form a cone. This cone can be conceived of as a bundle of individual rays, each reaching out to "feel" things visually and, on that basis, to locate and define them in space by reference to the vertex at the eye's center. But there is more to seeing than spatial perception. Color and luminosity, which are all but ignored by Euclid, seem not only integral but fundamental to sight. Recognizing this point, Ptolemy based his account of vision on color perception. Understood as a real and inherent quality of external objects, color, for Ptolemy, is what makes them visible. But, on its own, it cannot be seen; it needs the added power of light, which acts as a catalytic agent for vision. Seeing therefore begins with the primitive grasp of color by visual flux when it touches a properly illuminated object. Transmitted radially back through the cone of flux to the eye, the resulting color impression gives rise to the perception of spatial characteristics, such as size, shape, and distance, which in turn gives rise to a perception of the object as a whole. For Ptolemy, then, color perception is absolutely primal; all other perceptions are derivative.

The second model of vision harks back to Alhacen (9651040) and his Perspectivist disciples, Roger Bacon (fl. c. 1265), Witelo (fl. c. 1275), and John Pecham (fl. c. 1280). Rejecting visual rays as functionally pointless, these theorists raised light to primacy in the visual process, supposing it to be an intrinsic quality of self-luminous or illuminated bodies. Each point of light on the surface of such bodies is a source of radiation in its own right, spreading outward in all directions in a process of self-replication. The resulting sphere of propagation can be analytically resolved into individual rays, along which point forms of the original light are transmitted. Color, too, is an intrinsic property of bodies. Yet although they are ontologically distinct, light and color are functionally inseparable. Both must be present in objects if they are to be seen, so what actually radiates from them is luminous color. Thus, like Ptolemy, the Perspectivists viewed luminous color as primal for sight.

Unlike Ptolemy, the Perspectivists gave a detailed account of how the optic complex contributes to vision. The eye itself, they assumed, is a sphere. Toward its front lies the crystalline lens, whose anterior surface is concentric with the eye as a whole. The space behind it is filled with vitreous humor, which is optically denser than the glacial humor occupying the lens. At the very back, directly in line with the center of the pupil and the center of the eye, lies the hollow optic nerve, which reaches from the eye to the forefront of the brain. A conduit for visual spirit manufactured in the brain, this nerve transmits the spirit to the lens and thereby sensitizes it. The anterior surface of the lens, meanwhile, is bombarded from all directions by point forms of luminous color radiating from external objects. Because of its visual sensitivity, though, the lens feels only those color forms that strike it orthogonally and thus selects out a formal representation of the object in point-to-point correspondence with it. The composite of all the rays linking the object and its formal representation on the lens's surface creates a cone of radiation with its base in the object and its vertex at the center of the eye. Mathematically equivalent to Ptolemy's visual cone, this radiative cone serves much the same function as the basis for spatial perception.

The lens's ability to select coherent visual representations is also optically determined. As a refractive body, the lens allows only those rays that strike it orthogonally to pass straight through toward the center of the eye. Before they reach that point, they are refracted at the back surface of the lens so as to channel the visual representation in proper upright order into the hollow optic nerve. Conveyed by the spirit perfusing this nerve, the visual representation eventually reaches the brain, where it is subject to perceptual scrutiny. From this scrutiny arises a more abstract perceptual representation of the object according to all its visible attributes. More abstract yet is the ensuing conceptual representation, by means of which we perceive the object as a specific or general type. Each succeeding representation is a virtual likeness of its predecessor, much as a painting is a likeness of its subject. Hence, from start to finish, visual perception unfolds in a succession of virtual replications that ensures a fundamental correspondence between objective reality and our mind's-eye picture of it.

The Perspectivists were thus convinced that vision is veridical under the right conditionsadequate light, a healthy eye, and so forth. But under the wrong conditions, sight can err. Reflection and refraction offer two specific and egregious examples. In both cases there is a clear disparity between reality and appearance, insofar as things always appear displaced and often distorted in mirrors and refracting media. Accordingly, the Perspectivists were at pains to reconcile appearance with reality on the basis of ray geometry. The result was an elaborate analysis of image formation and distortion in mirrors and refracting media based on two principles: the law of equal angles for reflection and the cathetus rule of image location for reflection and refraction. According to this rule, the image of any point object seen in a mirror will lie at the intersection of the extended line of reflection, which constitutes the line of sight, and the perpendicular dropped from the object point to the surface of reflection. Neverthelessand this point is crucialthe ultimate goal of this analysis was not to understand how light interacts with reflecting and refracting surfaces. It was to understand how things are perceived or, rather, misperceived by means of such surfaces. Perspectivist optics, in short, was "subjective," not "objective," in its analytic focus.

Not all optical phenomena are subjective, though. Long before the Renaissance, it was known that spherical and parabolic concave mirrors can gather incoming light rays to a point or spot where tinder will ignite. By at least 1300, moreover, it was known that convex lenses can correct presbyopia. And while this could be explained away through refractive magnification, the correction of myopia by concave lenses (known by the mid-fifteenth century at latest) could not. Not only do such lenses not magnify what is seen through them; they actually reduce it. In addition, by the mid- to late-sixteenth century, it had become relatively common knowledge that concave mirrors, convex lenses, and pinhole openings (the camera obscura) can project images onto a screen. Lying not "in" the mirror or lens but outside it, such images make little or no sense according to Perspectivist theory, in which all images are virtual, or subjective.

Perhaps that is why such phenomena were essentially disregarded within academic circles, where Perspectivist theory predominated. Yet over the fifteenth and sixteenth centuries, those same phenomena captured the attention of artists, instrument makers, and leisured amateurs who, unlike their academic confreres, tended to be less theoretical than pragmatic, even instrumentalist, in their orientation. Growing interest in the focusing properties of lenses and mirrors over the sixteenth century bears directly on this point. An early example of this interest can be found in Francesco Maurolyco's study of the lenticular correction of presbyopia and myopia. Published posthumously in the Photismi de lumine (1611), but dating to the mid-sixteenth century, this study is noteworthy in two respects. First, its theoretical underpinnings are thoroughly Perspectivist. Although he felt free to adjust the model slightly by having the visual image selected from a particular sheaf of oblique rather than perpendicular rays, Maurolyco had no doubt that the selection itself occurred at the crystalline lens. Second, despite his reliance on Perspectivist principles, albeit somewhat modified, Maurolyco couched his explanation in terms not of light radiation but of its apparent antithesis, visual radiation. While such conflation may seem illogical to us, it was anything but for Maurlyco and his pragmatically oriented contemporaries. After all, light rays and visual rays are mathematically equivalent, so, as far as pure geometrical analysis is concerned, they are interchangeable. In many ways, in fact, the visual ray model is preferable, because it is both conceptually and mathematically simpler.

Maurolyco's pioneering study of lenses manifests a subtle but important change in attitude toward reflection and refraction during the later Renaissance. Before, within the Perspectivist framework, both had been regarded as sources of misperception. Now they were looked to as a means not of deluding sight but of rectifying or improving it. To this end, a succession of thinkers after Maurolyco, Giambattista della Porta (15351615) foremost among them, turned their attention to image magnification in convex lenses and concave mirrors in the hope of constructing an effective telescopic device. Although they failed in this, they at least succeeded in nudging the study of lenses and mirrorsas well as of their focusing propertiestoward the mainstream of optical analysis. It would be up to Kepler and Galileo to bring this study fully into the mainstream during the first few years of the 1600s.

THE KEPLERIAN TRANSITION

Early in his effort to determine the orbit of Mars, Kepler realized that in order to ensure the accuracy of his observational data, he had to address a variety of optical issues involving the camera obscura and atmospheric refraction. That in turn brought him to a close, critical scrutiny of Perspectivist theory, the results of which he published in 1604 in a wide-ranging critique entitled Ad Vitellionem paralipomena (Supplement to Witelo). Of particular interest is his account of retinal imaging in chapter five. Kepler began by supposing that the crystalline lens, like any other convex lens, is a refractive body and nothing more. Using a water-filled glass sphere to represent the lens, he examined how light passes through it to be brought to focus on the other side. He was thus led to conclude in the end that the eye acts like a camera, the pupil forming a diaphragm and the lens focusing all the rays passing through it from a given spot on the external object to a given spot on the retina. In this way, the light from all the spots on the surface of the object are projected to corresponding spots on the retina to form an inverted image, or "painting," of the object at the back of the eye.

At a superficial level, all Kepler did was displace the visual image from the front to the back of the eye, but at a deeper level he did far more than that. For a start, by doing away with the Perspectivist cone of radiation, Kepler did away with the center of sight as an essential reference point for optical analysis. Furthermore, being "real," not virtual, Kepler's image is publicit is there for anyone, not just the perceiver, to see. Worse, that image is inverted, not upright like its Perspectivist counterpart. Worse yet, it is too large to pass through the optic nerve to the brain for perceptual scrutiny. How, then, do such images give rise to visual perception? Kepler's response was to shunt the problem from optics to natural philosophy, arguing that the domain of optics extends no further than the retina. Opticians, in short, must restrict their study to the outward, physical manifestations of light alone. Its inward, perceptual manifestations are no longer their business.

Within six years of the publication of Kepler's account of retinal imaging, Galileo had fulfilled the hopes of earlier optical researchers by constructing a telescope that consisted of a convex objective and a concave eyepiece. Magnifying at least twenty times, this instrument had adequate resolution to allow a fairly distinct view of the four largest satellites of Jupiter. Published in the Sidereus Nuncius of 1610, news of this invention reached Kepler, who was eager to know precisely how it worked. His examination of the Galileian telescope led him to a rigorous geometrical analysis of lenses and lens combinations based solely on focal points. Among the results of that analysis, which appeared in the Dioptrice of 1611, was the design for a new kind of telescope whose objective and eyepiece were both convex. Technical details aside, Kepler accomplished two crucial things with this work. First, he brought refraction to the fore as a central concern for subsequent optical thinkers. Second, by stripping optics of its perceptual and epistemological entailments, he put the analytic focus squarely on light.

POST-KEPLERIAN DEVELOPMENTS

Having divorced the analysis of light from the analysis of sight, Kepler set the stage for a radical transformation of optics based on the mechanization of light. The key figure in this transformation was René Descartes, whose ideas about light and color took published form in the Dioptrique of 1637. According to Descartes, all light sources consist of infinitesimal particles clumped together so tightly as to form a virtual continuum. These clumps rotate swiftly, imparting a strong centrifugal tendency to the particles on their surface. But every light source is embedded in an ethereal medium composed of tiny spherical particles that are perfectly inelastic and contiguous. Instead, therefore, of flying off, the surface particles of the light source can only push against the unyielding ethereal envelope. The result is an outward impulse propagated instantaneously in all directions through it. This impulse is lightor, rather, what we perceive as lightand each individual line of impulse constitutes a "ray." What we perceive as transparency is nothing more than the capacity of ether particles to transmit light impulses. Color, for its part, is a function of spin imparted to the ethereal spheres by those impulses. The faster the spin, the more vivid the color as it verges from blue toward redor, rather, what we perceive as blue and red. The epistemological implications of this account are clear. Since physical light and its perceptual effect are absolutely different in kind, there is no meaningful way of linking them through virtual representation. "Red" and "bright" are therefore not objectively real. They are epiphenomenal, mere figments of our imagination.

Light many not actually be a projectile for Descartes, but it acts just like one. Accordingly, as a case of virtual motion along a virtual trajectory, light radiation must follow the laws of actual motion. This notion underlies Descartes's "proof" for the sine law of refraction, which is based on two fundamental principles: that, in rebounding from a reflective surface or penetrating a refractive medium, light loses none of its virtual motion, or "speed," along the horizontal, and that in penetrating a denser refractive medium, light gains virtual motion, or "speed," in proportion to the density. From this it follows that when light passes from one refractive medium to another, the ratio of the sines of the angle of incidence and the angle of refraction will be constant.

Descartes's account of light enjoyed a mixed reception. The "Schoolmen," who clung to medieval theory, rejected it outright. Among those who accepted it, some, like Robert Hooke, took it more or less at face value. Others accepted it on principle, realizing nonetheless that it was deeply flawed. The most glaring problem, of course, is the apparent contradiction in supposing that instantaneously transmitted light impulses can somehow vary in virtual motion or "speed." One obvious response to this problem is to assume that light radiation involves actual rather than virtual motion (an assumption that was eventually vindicated by Olaus Roemer's demonstration in 1679 that light takes time to travel). This is the tack Christiaan Huygens took in the 1670s. Assuming with Descartes that light consists of impulses transmitted through contiguous particles of ether, Huygens parted ways with him by making those particles elastic rather than inelastic. He proposed, therefore, that the impulse passed into the ether causes its constituent particles to contract and expand in succession, the result being a spherical wave front of condensations and rarefactions passing outward seriatim from the light source. To justify this longitudinal wave model of light, Huygens used it to good effect in explaining double refraction in Iceland spar, a phenomenon first brought to light by Erasmus Bartholin in 1669.

While Descartes, Hooke, and Huygens placed the motion, whether virtual or real, in the ethereal medium, others placed it the light itself. By 1662, for instance, Pierre de Fermat perfected his least-time proof of the sine law, which treats light as a particle shooting through space. Upon entering a denser refractive medium, this particle is impeded and slowed down commensurately, so that of all possible trajectories the particle could follow, the one dictated by the sine law takes the shortest time to traverse. The crucial turn in the evolution of a particle theory of light came with the publication of Newton's first paper on light and color in 1672. There Newton demonstrated experimentally that color is not a modification of white light, as Descartes would have it. On the contrary, being composed of all the colors in the prismatic spectrum, white light is a modification of color. Newton's eventual explanation of this fact rested on the supposition that each color is associated with a particle of a specific size. Building on this supposition in the Opticks of 1704, Newton developed a coherent analysis of light and color based on the interaction of color particles with gross matter as well as with exquisitely elastic ether particlesall such interactions being governed by attractive and repulsive forces. On this basis, Newton was able to explain an astonishing array of optical phenomena, ranging from simple reflection and refraction to double refraction, the formation of colored rings in thin glass plates ("Newton's Rings"), and even diffraction. With the appearance of Newton's Opticks, the theoretical lines were drawn for the rest of the eighteenth century. Huygens's longitudinal wave theory was not abandoned altogether, but because of its superior explanatory power, Newton's particle theory held sway until the early nineteenth century, when transverse waves became the wave of the future for optics.

Along with these theoretical developments, the seventeenth and eighteenth centuries witnessed a number of significant technical advances centering on telescopy and microscopy. The telescope, of course, found its first major publicists in Galileo and Kepler. Its close cousin the compound microscope found its key publicists somewhat later, first with the appearance of Robert Hooke's Micrographia in 1665 and subsequently with the observations of Jan Swammerdam and Antoni van Leeuwenhoek. For both instruments, however, resolution was a serious problem, and although it was mitigated somewhat as lenses with greater focal lengths were produced to give greater magnification, the resulting increase in telescope length narrowed the field of view.

The two main obstacles to proper resolution are spherical and chromatic aberration. The first of these stems from the fact that spherical lenses (as well as spherical concave mirrors) do not bring light to true focus. This problem inspired both Kepler and Descartes to seek the precise curvature that would bring parallel rays to focus at a single point, Descartes basing his analysis on the newly established sine law of refraction. As Descartes eventually proved, either a plano-hyperboloidal or spherico-ellipsoidal lens will suffice, hence the continuing effort during the middle decades of the seventeenth century to grind plano-hyperboloidal lenses. As promising as that expedient may have been in theory, it was far less so in practice, and the effort was eventually abandoned as hopeless. Chromatic aberration went unrecognized until Newton realized that lenses have a prismatic effect that disperses the light according to color, creating a sort of halo effect on telescopic images. To overcome this effect, he designed a reflecting telescope in which a concave spherical mirror serves as the objective. In fact, he constructed such a telescope and presented it to the Royal Society in 1671. But here, too, promise outstripped practicality, because it was all but impossible to keep the mirror from tarnishing or losing its proper shape.

The upshot was that over the later seventeenth and early eighteenth century, efforts were concentrated on improving the magnification of refracting telescopes and finding ways to widen the field of view in compensation. In addition, micrometers were added for greater observational precision, so that by the 1720s it was within around one second of arc. Eventually, however, the unwieldiness of such long telescopes coupled with improvements in the manufacture of concave mirrors led in the mid-eighteenth century to a renewed focus on reflecting telescopes. Steady improvements in such telescopes during the second half of the eighteenth century culminated with William Herschel's discovery of Uranus in 1781.

See also Astronomy ; Camera Obscura ; Descartes, René ; Galileo Galilei ; Hooke, Robert ; Huygens Family ; Kepler, Johannes ; Leeuwenhoek, Antoni van ; Newton, Isaac ; Scientific Instruments .

BIBLIOGRAPHY

Lindberg, David C. Theories of Vision from Al-Kindi to Kepler. Chicago, 1981.

Park, David. The Fire within the Eye. Princeton, 1997.

Ronchi, Vasco. Optics: The Science of Vision. Translated by Edward Rosen. New York, 1991.

Sabra, A. I. Theories of Light from Descartes to Newton. Cambridge, U.K., 1981.

Shapiro, Alan E. Fits, Passions, and Paroxysms: Physics, Method, and Chemistry and Newton's Theories of Colored Bodies and Fits of Easy Reflection. Cambridge, U.K., 1993.

Simon, Gérard. Archéologie de la vision. Paris, 2003.

Smith, A. Mark. "Alhacen's Theory of Visual Perception." Transactions of the American Philosophical Society 91 (1991): 45.

. "Descartes's Theory of Light and Refraction." Transactions of the American Philosophical Society, 77.3 (1987).

Van Helden, Albert. "The Invention of the Telescope." Transactions of the American Philosophical Society 67 (1977): 4.

Wolf-Devine, Celia. Descartes on Seeing. Carbondale, Ill. 1993.

A. Mark Smith

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optics

optics Branch of physics concerned with the study of light and its behaviour. Fundamental aspects are the physical nature of light, both as a wave phenomenon and as particles (photons), and the reflection, refraction (bending), and polarization (restricting vibrations to one direction) of light. Optics also involves the study of mirrors and lens systems and of optically active chemicals and crystals that polarize light. See also polarized light

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optics

optics, scientific study of light. Physical optics is concerned with the genesis, nature, and properties of light; physiological optics with the part light plays in vision; and geometrical optics with the reflection and refraction of light as encountered in the study of the mirror and the lens.

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optics

op·tics / ˈäptiks/ • pl. n. [usu. treated as sing.] the scientific study of sight and the behavior of light, or the properties of transmission and deflection of other forms of radiation.

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Optics

Optics

Electromagnetic waves

Wavelength, frequency, and the speed of light

Reflection and refraction

Resources

Optics is the branch of physics that is concerned with visible light and its properties. Physicists who focus on optics study the properties of light. Engineers also deal with optics when developing and making things such as telescopes, eyeglasses, cameras, microscopes, prisms, and various lenses for many purposes. Besides, visible light, optics also studies the invisible parts of the electromagnetic spectrum. They also apply these properties to phenomena such as color, mirrors, and lenses. Geometrical optics treats light phenomena (e.g., the determination of focal points, image characteristics, etc.) through calculations derived from the geometry of rays and similar triangles.

Ancient Greek philosophers were the first to study the properties of light. They theorized that light was made up of tiny particles that could enter the eye. The idea of the particulate nature of light was widely accepted well into the late eighteenth century. A few philosophersand later scientists from Greek philosopher Empedocles (490430 BC) to Dutch scientist Christian Huygens (16291695), argued that light was actually a wave.

Following English physicist Sir Isaac Newtons (16421727) 1704 publication of Optics, strength for the wave interpretation of light increased. In 1800, the dual nature of light was demonstrated conclusively by the classic double slit experiment of English physicist and physician Thomas Young (17731829). By the nineteenth century, the wave theory of light was widely accepted. In 1905 GermanAmerican physicist Albert Einsteins (18791955) photoelectric theory asserted that light behaves both as a particle and a wave.

Einsteins work extended German physicist Maxwell Plancks (18581947) concept of energy quantization to electromagnetic radiation.

Electromagnetic waves

Light is a form of electromagnetic radiation that travels in a wave with both electric and magnetic behavior. Light, therefore, propagates as an electromagnetic wave. Visible light represents one portion of a spectrum of electromagnetic waves that exist over a range of frequencies and wavelengths. Other electromagnetic waves include radio waves, microwaves, and x rays.

Light waves are transverse waves that oscillate perpendicular to their direction of travel. Waves can be vertically polarized and horizontally polarized so that the polarized light oscillates in one direction. The light from a common light source, such as a light bulb or the Sun, is not polarized. Light waves originating from these sources can oscillate at any orientation. When the light passes through a polarizing filter, such as polarized sunglasses, it exits as polarized light. The filter only allows passage of light waves oscillating in a predetermined plane.

Wavelength, frequency, and the speed of light

The relationship between the frequency (the number of wave crests that pass by a certain point in a given amount of time) and wavelength for electromagnetic waves is defined by the formula, c = λ f, where c is the speed of light, λ the wavelength in meters, and f equals the frequency in cycles per second. For example, the highest energy wavelength detectable by the human eye is generally determined to be 3.80 x 107 m. Rewriting the formula c = λ f as f = c/ λ yields (3.00 x 108 m/s / 3.80 x 107 m) = 7.9 x 1014 Hz for a frequency of the wave.

The inverse relationship between wavelength and frequency means that as wavelengths increase, frequency decreases. Because the frequency of a photon or electromagnetic wave is directly proportional to the energy of the photon or wave, the higher the frequency of the photon or wave, the greater the energy state of the photon or wave. For this reason, within the visible spectrum, shorter wavelength blue light is more energetic than longer wavelength red light (i.e., photons and electromagnetic [EM] waves with frequencies and wavelengths in the red portion of the spectrum).

Newton was the first scientist to study color. He passed sunlight through a prism and found that it could be separated into beams of light of different colors. He showed that visible light actually consists of red, orange, yellow, green, blue, and violet light. Each of these colors corresponds to a particular frequency and wavelength of light. Newton passed the individual color bands produced by the prism through a second prism. This second prism re-combined the individual bands and the light exited the prism as white light. This showed that white light is actually the combination of all of the colors of the spectrum.

The color of an object is due to the frequencies (and corresponding wavelengths) of light absorbed by the object. Most objects absorb the majority of the frequencies of light. Any frequencies that are not absorbed by the object are reflected, giving the object a particular color. If an object absorbs all light except the frequencies found in the red region of the spectrum, the object appears red. Red light is reflected off the object. White is actually not a color, but a combination of all colors, occurring when all frequencies of light are reflected. Likewise, black is actually the absence of reflected light, occurring when all frequencies of light are absorbed.

Light waves exhibit constructive and destructive interference patterns. Constructive interference occurs when two or more light waves meet in phase (e.g., wave crests meeting wave crests) and usually results in a more intense or bright resulting light. When the light waves meet out of phase (e.g., when the wave crests of one wave cancel the wave troughs of another wave), destructive interference takes place and light intensity is reduced or the light is negated.

The concept of interference is important for understanding the phenomena of diffraction. Youngs double-slit interference experiment is a classic explanation for diffraction, which is the bending of light as it passes around an object. Young made two small slits relatively close to each other on a dark board. When he shined a light through the slits and observed the light on a screen, he noticed that the light did not pass directly though in two straight lines. Instead, there was a pattern of alternating bright and dark bands of light. This resulted from the light waves fanning outdiffractingas they passed through the barrier slits, much like water ripples when it passes from a small opening into a larger body of water. Because light waves were passing through two slits, two fans were created that overlapped at certain points. Some of these points experienced destructive interference, while others were constructive; thus leading to the alternating bands of light. The dark bands occurred when light waves canceled each other out.

Reflection and refraction

Other phenomena associated with light include reflection and refraction. Light is reflected when the

light waves bounce off an object to travel in a new direction. A surface that causes light to bounce back is called a reflective surface. A mirror is an example of a reflective surface. The angle an out-going light ray (i.e., stream of photons or wave path) makes with a reflective surface will be equal to the angle of the incoming light ray. To an observer, a reflected light ray will appear to come from behind the reflecting surface. For example, when a person stands in front of a mirror, they will see an image of themselves that appears to be behind the mirror. Because the image appears to originate from an imaginary point, the image is called a virtual image. A virtual image created by a mirror is the same size as the original object.

Refraction can occur when light travels through one medium into another. The velocity of light is different for various materials. For instance, the velocity of light in air is slower than its velocity in vacuum and slower still in glass or plastic, for example. Under the correct circumstances, the light ray will be refracted back into the original material. In a sense, the light ray reflects off the boundary. For example, if a waterproof flashlight is held in a bathtub of water at different angles, a particular angle can be found where the beam does not escape the water to shine light through the air above the water surface. The light is refracted at the surface of the water back into the water instead of being passed through the water and into the air. This angle is called the critical angle. Any angle beyond the critical angle will cause total internal reflection.

Optical fibers, also called light pipes, utilize this phenomenon. Light travels through the transparent fibers by a series of total internal reflections, much like a rubber ball would bounce through a pipe. Fiber optics have many different important uses today. Mechanics use optical fibers to shine light deep into engines. Surgeons use them to see inside a patients body. Optical fibers are also used in communications because they are less bulky and more inexpensive than copper cables. Information in these fibers is carried by light instead of electrical current.

Another form of light that has become indispensable in society is the laser. The light in lasers results from photons emitted by highly excited atoms returning to their ground state. The photons are harnessed between two mirrors where they continue to collide until they collectively exit in one direction at a specific wavelength. Laser light is a very precise, specific wavelength that can be altered to match the absorption of almost any substance. The laser light will only damage materials whose absorption band matches the lasers wavelength. This controlled intensity makes the laser a handy tool for several applications ranging from surgery to reading compact disks (CDs) and digital versatile discs (DVDs).

See also Electromagnetic spectrum; Fluorescent light.

Resources

BOOKS

Chang, William Shen-chie. Principles of Lasers and Optics. Cambridge, UK, and New York: Cambridge University Press, 2005.

Csele, Mark. Fundamentals of Light Sources and Lasers. Hoboken, NJ: J. Wiley, 2004.

Hecht, Eugene. Optics, 4th ed. Boston: Addison-Wesley Publishing, 2001.

Loudon, Rodney. The Quantum Theory of Light, 3rd. ed. New York: Oxford University Press, 2002.

Rieke, George. Detection of Light: From the Ultraviolet to the Submillimeter. New York: Cambridge University Press, 2002.

OTHER

Beeson, Steve, Arizona State University. Patterns in Nature, Light and Optics <http://acept.la.asu.edu/PiN/mod/light/pattLightOptics.html> (accessed November 18, 2006).

Jennifer McGrath

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Optics

Optics

Optics is the branch of physics that is concerned with visible light and its properties. Physicists who focus on optics study the properties of light. They also apply these properties to phenomena such as color , mirrors , and lenses. Geometrical optics treats light phenomena (e.g., the determination of focal points, image characteristics, etc.) through calculations derived from the geometry of rays and similar triangles.

Ancient Greek philosophers were the first to study the properties of light. They theorized that light was made up of tiny particles that could enter the eye . The idea of the particulate nature of light was widely accepted well into the late eighteenth century. A few philosophers—and later scientists from the Greek philosopher Empedocles (490–430 b.c.) to the Dutch scientist Christian Huygens (1629-1695), argued that light was actually a wave.

Following English physicist Sir Isaac Newton's (1642–1727) 1704 publication of Optics, strength for the wave interpretation of light increased. In 1800, the dual nature of light was demonstrated conclusively by the classic double slit experiment of Thomas Young. By the nineteenth century, the wave theory of light was widely accepted. In 1905 Albert Einstein's (1879–1955) photoelectric theory asserted that light behaves both as a particle and a wave.

Einstein's work extended German physicist Maxwell Planck's (1858–1947) concept of energy quantization to electromagnetic radiation .


Electromagnetic waves

Light is a form of electromagnetic radiation that travels in a wave with both electric and magnetic behavior. Light, therefore, propagates as an electromagnetic wave. Visible light represents one portion of a spectrum of electromagnetic waves that exist over a range of frequencies and wavelengths Other electromagnetic waves include radio waves , microwaves, and x rays .

Light waves are transverse waves that oscillate perpendicular to their direction of travel. Waves can be vertically polarized and horizontally polarized so that the polarized light oscillates in one direction. The light from a common light source, such as a light bulb or the Sun , is not polarized. Light waves originating from these sources can oscillate at any orientation. When the light passes through a polarizing filter, such as polarized sunglasses, it exits as polarized light. The filter only allows passage of light waves oscillating in a predetermined plane .


Wavelength, frequency, and the speed of light

The relationship between the frequency (the number of wave crests that pass by a certain point in a given amount of time) and wavelength for electromagnetic waves is defined by the formula, c = λ f, where c is the speed of light, λ the wavelength in meters, and f equals the frequency in cycles per second. For example, the highest energy wavelength detectable by the human eye is generally determined to be 3.80 x 10-7 m. Rewriting the formula c = λ f as f = c / λ yields (3.00 x 108 m/s / 3.80 x 10-7 m) = 7.9 x 1014 Hz for a frequency of the wave.

The inverse relationship between wavelength and frequency means that as wavelengths increase, frequency decreases. Because the frequency of a photon or electromagnetic wave is directly proportional to the energy of the photon or wave,the higher the frequency of the photon or wave, the greater the energy state of the photon or wave. For this reason, within the visible spectrum, shorter wavelength blue light is more energetic than longer wavelength red light (i.e., photons and em waves with frequencies and wavelengths in the red portion of the spectrum).

Newton was the first scientist to study color. He passed sunlight through a prism and found that it could be separated into beams of light of different colors. He showed that visible light actually consists of red, orange, yellow, green, blue, and violet light. Each of these colors corresponds to a particular frequency and wavelength of light. Newton passed the individual color bands produced by the prism through a second prism. This second prism re-combined the individual bands and the light exited the prism as white light. This showed that white light is actually the combination of all of the colors of the spectrum.

The color of an object is due to the frequencies (and corresponding wavelengths) of light absorbed by the object. Most objects absorb the majority of the frequencies of light. Any frequencies that are not absorbed by the object are reflected, giving the object a particular color. If an object absorbs all light except the frequencies found in the red region of the spectrum, the object appears red. Red light is reflected off of the object. White is actually not a color, but a combination of all colors, occurring when all frequencies of light are reflected. Likewise, black is actually the absence of reflected light, occurring when all frequencies of light are absorbed.

Light waves exhibit constructively and destructive interference patterns. Constructive interference occurs when two or more light waves meet in phase (e.g., wave crests meeting wave crests) and usually results in a more intense or bright resulting light. When the light waves meet out of phase (e.g., when the wave crests of one wave cancel the wave troughs of another wave), destructive interference takes place and light intensity is reduced or the light is negated.

The concept of interference is important for understanding the phenomena of diffraction . Young's double-slit interference experiment is a classic explanation for diffraction, which is the bending of light as it passes around an object. Young made two small slits relatively close to each other on a dark board. When he shined a light through the slits and observed the light on a screen, he noticed that the light did not pass directly though in two straight lines. Instead, there was a pattern of alternating bright and dark bands of light. This resulted from the light waves fanning out-diffracting-as they passed through the barrier slits, much like water ripples when it passes from a small opening into a larger body of water. Because light waves were passing through two slits, two fans were created that overlapped at certain points. Some of these points experienced destructive interference, while others were constructive, thus leading to the alternating bands of light. The dark bands occurred when light waves canceled each other out.


Reflection and refraction

Other phenomena associated with light include reflection and refraction. Light is reflected when the light waves bounce off of an object to travel in a new direction. A surface that causes light to bounce back is called a reflective surface. A mirror is an example of a reflective surface. The angle an out-going light ray (i.e., stream of photons or wave path) makes with a reflective surface will be equal to the angle of the incoming light ray. To an observer, a reflected light ray will appear to come from behind the reflecting surface. For example, when a person stands in front of a mirror, they will see an image of themselves that appears to be behind the mirror. Because the image appears to originate from an imaginary point, the image is called a virtual image. A virtual image created by a mirror is the same size as the original object.

Refraction can occur when light travels through one medium into another. The velocity of light is different for various materials. For instance, the velocity of light in air is slower than than its velocity in vacuum and slower still in glass or plastic. Under the right circumstances, the light ray will be refracted back into the original material. In a sense, the light ray reflects off the boundary. For example, if a waterproof flashlight is held in a bathtub of water at different angles, a particular angle can be found where the beam does not escape the water to shine light through the air above the water surface. The light is refracted at the surface of the water back into the water instead of being passed through the water and into the air. This angle is called the critical angle. Any angle beyond the critical angle will cause total internal reflection.

Optical fibers, also called light pipes, utilize this phenomenon. Light travels through the transparent fibers by a series of total internal reflections , much like a rubber ball would bounce through a pipe. Fiber optics have many different important uses today. Mechanics use optical fibers to shine light deep into engines. Surgeons use them to see inside a patient's body. Optical fibers are also used in communications because they are less bulky and more inexpensive than copper cables. Information in these fibers is carried by light instead of electrical current.

Another form of light that has become indispensable in society is the laser . The light in lasers results from photons emitted by highly excited atoms returning to their ground state. The photons are harnessed between two mirrors where they continue to collide until they collectively exit in one direction at a specific wavelength. Laser light is a very precise, specific wavelength that can be altered to match the absorption of almost any substance. The laser light will only damage materials whose absorption band matches the lasers wavelength. This controlled intensity makes the laser a handy tool for several applications ranging from surgery to reading compact disks.

See also Electromagnetic spectrum; Fluorescent light.

Resources

books

Hecht, Eugene. Optics. 4th ed. Boston: Addison-Wesley Publishing, 2001.

Loudon, Rodney. The Quantum Theory of Light. 3rd. ed. New York: Oxford University Press, 2002.

Rieke, George. Detection of Light: From the Ultraviolet to theSubmillimeter. New York: Cambridge University Press, 2002.

other

Beeson, Steve. Arizona State University. "Patterns in Nature, Light and Optics" [cited March, 10, 2003] <http://acept.la.asu.edu/PiN/mod/light/pattLightOptics.html>.

Jennifer McGrath

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"Optics." The Gale Encyclopedia of Science. . Encyclopedia.com. 13 Nov. 2018 <https://www.encyclopedia.com>.

"Optics." The Gale Encyclopedia of Science. . Encyclopedia.com. (November 13, 2018). https://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/optics

"Optics." The Gale Encyclopedia of Science. . Retrieved November 13, 2018 from Encyclopedia.com: https://www.encyclopedia.com/science/encyclopedias-almanacs-transcripts-and-maps/optics

Learn more about citation styles

Citation styles

Encyclopedia.com gives you the ability to cite reference entries and articles according to common styles from the Modern Language Association (MLA), The Chicago Manual of Style, and the American Psychological Association (APA).

Within the “Cite this article” tool, pick a style to see how all available information looks when formatted according to that style. Then, copy and paste the text into your bibliography or works cited list.

Because each style has its own formatting nuances that evolve over time and not all information is available for every reference entry or article, Encyclopedia.com cannot guarantee each citation it generates. Therefore, it’s best to use Encyclopedia.com citations as a starting point before checking the style against your school or publication’s requirements and the most-recent information available at these sites:

Modern Language Association

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The Chicago Manual of Style

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American Psychological Association

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Notes:
  • Most online reference entries and articles do not have page numbers. Therefore, that information is unavailable for most Encyclopedia.com content. However, the date of retrieval is often important. Refer to each style’s convention regarding the best way to format page numbers and retrieval dates.
  • In addition to the MLA, Chicago, and APA styles, your school, university, publication, or institution may have its own requirements for citations. Therefore, be sure to refer to those guidelines when editing your bibliography or works cited list.