Yet, within that realm are an almost infinite array of hues that quite literally give color to the entire world of human experience. Light, of course, is more than color: it is energy, which travels at incredible speeds throughout the universe. From prehistoric times, humans harnessed light's power through fire, and later, through the invention of illumination devices such as candles and gas lamps. In the late nineteenth century, the first electric- powered forms of light were invented, which created a revolution in human existence. Today, the power of lasers, highly focused beams of high- intensity light, make possible a number of technologies used in everything from surgery to entertainment. HOW IT WORKSEarly Progress in Understanding of Light. The first useful observations concerning light came from ancient Greece. The Greeks recognized that light travels through air in rays, a term from geometry describing that part of a straight line that extends in one direction only. Upon entering some denser medium, such as glass or water, as Greek scientists noticed, the ray experiences refraction, or bending. Another type of incidence, or contact, between a light ray and any surface, is reflection, whereby a light ray returns, rather than being absorbed at the interface. The Greeks worked out the basic laws governing reflection and refraction, observing, for instance, that in reflection, the angle of incidence is approximately equal to the angle of reflection. Unfortunately, they also subscribed to the erroneous concept of intromission. Some 1,5. 00 years after the high point of Greek civilization, Arab physicist Alhasen (Ibn al- Haytham; c. Middle Ages, showed that light comes from a source such as the Sun, and reflects from an object to the eyes. The next great era of progress in studies of light began with the Renaissance (c. However, the most profound scientific achievements in this area belonged not to scientists, but to painters, who were fascinated by color, shading, shadows, and other properties of light. During the early seventeenth century, Galileo Galilei (1. German astronomer Johannes Kepler (1. Dutch physicist and mathematician Willebrord Snell (1. The Spectrum. Sir Isaac Newton (1. Though it was not as epochal as his contributions to mechanics, Newton's work in optics, an area of physics that studies the production and propagation of light, was certainly significant. In Newton's time, physicists understood that a prism could be used for the diffusion of light. The prevailing belief was that white was a single color like the others, but Newton maintained that it was a combination of all other colors. To prove this, he directed a beam of white light through a prism, then allowed the diffused colors to enter another prism, at which point they recombined as white light. Newton gave to the array of colors in visible light the term spectrum, (plural, . The term can be used for any set of characteristics for which there is a gradation, as opposed to an excluded middle. An ordinary light switch provides an example of a situation in which there is an excluded middle: there is nothing between .
This feature is not available right now. Please try again later. Donald Briscoe Dark Shadows Characters: Tom Jennings, Chris Jennings, Chris Collins, Tim Shaw. The Shadows 20 Golden Greats. EMTV 3 Apache/Man of Mystery /The Frightened City/Guitar Tango/Kon Tiki/Foot Tapper/ Genie with the light brown lamp/The Warlord/A place. The reasons for this arrangement, explained below in the context of the electromagnetic spectrum, were unknown to Newton. Not only did he live in an age that had almost no understanding of electromagnetism, but he was also a product of the era called the Enlightenment, when intellectuals (scientists included) viewed the world as a highly rational, ordered mechanism. His Enlightenment viewpoint undoubtedly influenced his interpretation of the spectrum as a set of seven colors, just as there are seven notes on the musical scale. In addition to the six basic colors listed above, Newton identified a seventh, indigo, between blue and violet. In fact, there is a noticeable band of color between blue and violet, but this is because one color fades into another. With a spectrum, there is a blurring of lines between one color and the next: for instance, orange exists at a certain point along the spectrum, as does yellow, but between them is a nearly unlimited number of orange- yellow and yellow- orange gradations. Indigo itself is not really a distinct color. But its inclusion in the listing of colors on the spectrum has given generations of students a handy mnemonic (memorization). Incidentally, there is something arbitrary even in the idea of six colors, or for that matter seven musical notes: in both cases, there exists a very large gradation of shades, yet also in both cases, the divisions used were chosen for practical purposes. Waves, Particles, and Other Questions Concerning Light. THE WAVE- PARTICLE CONTROVERSY BEGINS. Newton subscribed to the corpuscular theory of light: the idea that light travels as a stream of particles. On the other hand, Dutch physicist and astronomer Christiaan Huygens (1. During the century that followed, adherents of particle theory did intellectual battle with proponents of wave theory. Reflecting both the burgeoning awareness of the nation- state among Europeans, as well as Britons' sense of their own island as an entity separate from the European continent, particle theory had its strongest defenders in Newton's. According to Huygens, the appearance of the spectrum, as well as the phenomena of reflection and refraction, indicated that light was a wave. Newton responded by furnishing complex mathematical calculations which showed that particles could exhibit the behaviors of reflection and refraction as well. Furthermore, Newton challenged, if light were really a wave, it should be able to bend around corners. Yet, in 1. 66. 0, an experiment by Italian physicist Francesco Grimaldi (1. Passing a beam of light through a narrow aperture, or opening, Grimaldi observed a phenomenon called diffraction, or the bending of light. In view of the nationalistic character that the wave- particle debate assumed, it was ironic that the physicist whose work struck a particularly forceful blow against corpuscular theory was himself an Englishman: Thomas Young (1. Directing a light beam through two closely spaced pinholes onto a screen, Young reasoned that if light truly were made of particles, the beams would project two distinct points onto the screen. Instead, what he saw was a pattern of interference. Experiments in 1. Jean Bernard Leon Foucault (1. Based on studies of wave motion up to that time, Foucault's work added substance to the view of light as a wave. Foucault also measured the speed of light in a vacuum, a speed which he calculated to within 1% of its value as it is known today: 1. An understanding of just how fast light traveled, however, caused a nagging question dating back to the days of Newton and Huygens to resurface: how did light travel? All types of waves known to that time traveled through some sort of medium: for instance, sound waves were propagated through air, water, or some other type of matter. If light was a wave, as Huygens said, then it, too, must have some medium. Huygens and his followers proposed a weak theory by suggesting the existence of an invisible substance called ether, which existed throughout the universe and which carried light. Ether, of course, was really no answer at all. There was no evidence that it existed, and to many scientists, it was merely a concept invented to shore up an otherwise convincing argument. Then, in 1. 87. 2, Scottish physicist James Clerk Maxwell (1. His work led to the identification of a . This was the mode of particle interaction associated with electromagnetic force. The particulars of electromagnetic force, waves, and radiation are a subject unto themselves. As for the electromagnetic spectrum, it is treated at some length in an essay elsewhere in this volume, and the reader is encouraged to review that essay to gain a greater understanding of light and its place in the spectrum. In addition, some awareness of wave motion and related phenomena would also be of great value, and, for this purpose, other essays are recommended. In the present context, a number of topics relating to these larger subjects will be handled in short order, with a minimum of explanation, to enable a more speedy transition to the subject of principal importance here: light. ELECTROMAGNETIC WAVES. There is, of course, no obvious connection between light and the electromagnetic force observed in electrical and magnetic interactions. Yet, light is an example of an electromagnetic wave, and is part of the electromagnetic spectrum. The breakthrough in establishing the electromagnetic quality of light can be attributed both to Maxwell and German physicist Heinrich Rudolf Hertz (1. In his Electricity and Magnetism (1. Maxwell suggested that electromagnetic force. This appeared to be more than just a coincidence, and his findings led him to theorize that the electromagnetic interaction included not only electricity and magnetism, but light as well. Some time later, Hertz proved Maxwell's hypothesis by showing that electromagnetic waves obeyed the same laws of reflection, refraction, and diffraction as light. Hertz also discovered the photoelectric effect, the process by which certain metals acquire an electrical potential when exposed to light. He could not explain this behavior, and, indeed, there was nothing in wave theory that could account for it. Strangely, after more than a century in which acceptance of wave theory had grown, he had encountered something that apparently supported what Newton had said long before: that light traveled in particles rather than waves. The wave- Particle Debate Revisited. One of the modern physicists whose name is most closely associated with the subject of light is Albert Einstein (1. In the course of proving that matter is convertible to energy, as he did with the theory of relativity, Einstein predicted that this could be illustrated by accelerating to speeds close to that of light. Quantum theory and quantum mechanics are, of course, far too complicated to explain in any depth here.
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