The Discovery of Light

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Among these was that oxygen was a unique element, in that it was the only supporter of combustion and was also the element that lay at the basis of all acids. Davy, after having discovered sodium and potassium by using a powerful current from a galvanic battery to decompose oxides of these elements, turned to the decomposition of muriatic hydrochloric acid, one of the strongest acids known.

The products of the decomposition were hydrogen and a green gas that supported combustion and that, when combined with water, produced an acid. Davy concluded that this gas was an element, to which he gave the name chlorine, and that there was no oxygen whatsoever in muriatic acid. Acidity, therefore, was not the result of the presence of an acid-forming element but of some other condition. What else could that condition be but the physical form of the acid molecule itself? Davy suggested, then, that chemical properties were determined not by specific elements alone but also by the ways in which these elements were arranged in molecules.

In arriving at this view he was influenced by an atomic theory that was also to have important consequences for Faraday's thought. This theory, proposed in the 18th century by Ruggero Giuseppe Boscovich, argued that atoms were mathematical points surrounded by alternating fields of attractive and repulsive forces. A true element comprised a single such point, and chemical elements were composed of a number of such points, about which the resultant force fields could be quite complicated.

Molecules, in turn, were built up of these elements, and the chemical qualities of both elements and compounds were the results of the final patterns of force surrounding clumps of point atoms. One property of such atoms and molecules should be specifically noted: they can be placed under considerable strain, or tension, before the "bonds" holding them together are broken. These strains were to be central to Faraday's ideas about electricity. Faraday's second apprenticeship, under Davy, came to an end in By then he had learned chemistry as thoroughly as anyone alive.

He had also had ample opportunity to practice chemical analyses and laboratory techniques to the point of complete mastery, and he had developed his theoretical views to the point that they could guide him in his researches. There followed a series of discoveries that astonished the scientific world. Faraday achieved his early renown as a chemist. His reputation as an analytical chemist led to his being called as an expert witness in legal trials and to the building up of a clientele whose fees helped to support the Royal Institution. In he produced the first known compounds of carbon and chlorine, C2Cl6 and C2Cl4.

These compounds were produced by substituting chlorine for hydrogen in "olefiant gas" ethylene , the first substitution reactions induced.

Such reactions later would serve to challenge the dominant theory of chemical combination proposed by Jns Jacob Berzelius. In , as a result of research on illuminating gases, Faraday isolated and described benzene. In the s he also conducted investigations of steel alloys, helping to lay the foundations for scientific metallurgy and metallography. While completing an assignment from the Royal Society of London to improve the quality of optical glass for telescopes, he produced a glass of very high refractive index that was to lead him, in , to the discovery of diamagnetism.

In he married Sarah Barnard, settled permanently at the Royal Institution, and began the series of researches on electricity and magnetism that was to revolutionize physics. In Hans Christian rsted had announced the discovery that the flow of an electric current through a wire produced a magnetic field around the wire. Andr-Marie Ampre showed that the magnetic force apparently was a circular one, producing in effect a cylinder of magnetism around the wire.

No such circular force had ever before been observed, and Faraday was the first to understand what it implied. If a magnetic pole could be isolated, it ought to move constantly in a circle around a current-carrying wire. Faraday's ingenuity and laboratory skill enabled him to construct an apparatus that confirmed this conclusion.

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This device, which transformed electrical energy into mechanical energy, was the first electric motor. This discovery led Faraday to contemplate the nature of electricity. Unlike his contemporaries, he was not convinced that electricity was a material fluid that flowed through wires like water through a pipe. Instead, he thought of it as a vibration or force that was somehow transmitted as the result of tensions created in the conductor. One of his first experiments after his discovery of electromagnetic rotation was to pass a ray of polarized light through a solution in which electrochemical decomposition was taking place in order to detect the intermolecular strains that he thought must be produced by the passage of an electric current.

During the s he kept coming back to this idea, but always without result. In the spring of Faraday began to work with Charles later Sir Charles Wheatstone on the theory of sound, another vibrational phenomenon. He was particularly fascinated by the patterns known as Chladni figures formed in light powder spread on iron plates when these plates were thrown into vibration by a violin bow. Here was demonstrated the ability of a dynamic cause to create a static effect, something he was convinced happened in a current-carrying wire. He was even more impressed by the fact that such patterns could be induced in one plate by bowing another nearby.

Such acoustic induction is apparently what lay behind his most famous experiment. On August 29, , Faraday wound a thick iron ring on one side with insulated wire that was connected to a battery. He then wound the opposite side with wire connected to a galvanometer. What he expected was that a "wave" would be produced when the battery circuit was closed and that the wave would show up as a deflection of the galvanometer in the second circuit. He closed the primary circuit and, to his delight and satisfaction, saw the galvanometer needle jump. A current had been induced in the secondary coil by one in the primary.

When he opened the circuit, however, he was astonished to see the galvanometer jump in the opposite direction. Somehow, turning off the current also created an induced current in the secondary circuit, equal and opposite to the original current. This phenomenon led Faraday to propose what he called the "electrotonic" state of particles in the wire, which he considered a state of tension. A current thus appeared to be the setting up of such a state of tension or the collapse of such a state. Although he could not find experimental evidence for the electrotonic state, he never entirely abandoned the concept, and it shaped most of his later work.

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In the fall of Faraday attempted to determine just how an induced current was produced. His original experiment had involved a powerful electromagnet, created by the winding of the primary coil. He now tried to create a current by using a permanent magnet. He discovered that when a permanent magnet was moved in and out of a coil of wire a current was induced in the coil.

Magnets, he knew, were surrounded by forces that could be made visible by the simple expedient of sprinkling iron filings on a card held over them. Faraday saw the "lines of force" thus revealed as lines of tension in the medium, namely air, surrounding the magnet, and he soon discovered the law determining the production of electric currents by magnets: the magnitude of the current was dependent upon the number of lines of force cut by the conductor in unit time.

He immediately realized that a continuous current could be produced by rotating a copper disk between the poles of a powerful magnet and taking leads off the disk's rim and centre. The outside of the disk would cut more lines than would the inside, and there would thus be a continuous current produced in the circuit linking the rim to the centre. This was the first dynamo.

5th-3rd B.C.

It was also the direct ancestor of electric motors, for it was only necessary to reverse the situation, to feed an electric current to the disk, to make it rotate. While Faraday was performing these experiments and presenting them to the scientific world, doubts were raised about the identity of the different manifestations of electricity that had been studied. Were the electric "fluid" that apparently was released by electric eels and other electric fishes, that produced by a static electricity generator, that of the voltaic battery, and that of the new electromagnetic generator all the same?

Or were they different fluids following different laws? Faraday was convinced that they were not fluids at all but forms of the same force, yet he recognized that this identity had never been satisfactorily shown by experiment. For this reason he began, in , what promised to be a rather tedious attempt to prove that all electricities had precisely the same properties and caused precisely the same effects.

The key effect was electrochemical decomposition. Voltaic and electromagnetic electricity posed no problems, but static electricity did. As Faraday delved deeper into the problem, he made two startling discoveries. First, electrical force did not, as had long been supposed, act at a distance upon chemical molecules to cause them to dissociate. It was the passage of electricity through a conducting liquid medium that caused the molecules to dissociate, even when the electricity merely discharged into the air and did not pass into a "pole" or "centre of action" in a voltaic cell.

Second, the amount of the decomposition was found to be related in a simple manner to the amount of electricity that passed through the solution. These findings led Faraday to a new theory of electrochemistry. The electric force, he argued, threw the molecules of a solution into a state of tension his electrotonic state. When the force was strong enough to distort the fields of forces that held the molecules together so as to permit the interaction of these fields with neighbouring particles, the tension was relieved by the migration of particles along the lines of tension, the different species of atoms migrating in opposite directions.

The amount of electricity that passed, then, was clearly related to the chemical affinities of the substances in solution. These experiments led directly to Faraday's two laws of electrochemistry: 1 The amount of a substance deposited on each electrode of an electrolytic cell is directly proportional to the quantity of electricity passed through the cell.

Faraday's work on electrochemistry provided him with an essential clue for the investigation of static electrical induction. Since the amount of electricity passed through the conducting medium of an electrolytic cell determined the amount of material deposited at the electrodes, why should not the amount of electricity induced in a nonconductor be dependent upon the material out of which it was made? In short, why should not every material have a specific inductive capacity? Every material does, and Faraday was the discoverer of this fact.

By Faraday was able to bring forth a new and general theory of electrical action. Electricity, whatever it was, caused tensions to be created in matter. When these tensions were rapidly relieved i. Such substances were called conductors. In electrochemical processes the rate of buildup and breakdown of the strain was proportional to the chemical affinities of the substances involved, but again the current was not a material flow but a wave pattern of tensions and their relief.

Insulators were simply materials whose particles could take an extraordinary amount of strain before they snapped. Electrostatic charge in an isolated insulator was simply a measure of this accumulated strain. Thus, all electrical action was the result of forced strains in bodies. The strain on Faraday of eight years of sustained experimental and theoretical work was too much, and in his health broke down. For the next six years he did little creative science.

Not until was he able to pick up the thread of his researches and extend his theoretical views. Since the very beginning of his scientific work, Faraday had believed in what he called the unity of the forces of nature. By this he meant that all the forces of nature were but manifestations of a single universal force and ought, therefore, to be convertible into one another.

In he made public some of the speculations to which this view led him. A lecturer, scheduled to deliver one of the Friday evening discourses at the Royal Institution by which Faraday encouraged the popularization of science, panicked at the last minute and ran out, leaving Faraday with a packed lecture hall and no lecturer. On the spur of the moment, Faraday offered "Thoughts on Ray Vibrations. Many years later, Maxwell was to build his electromagnetic field theory upon this speculation.

When Faraday returned to active research in , it was to tackle again a problem that had obsessed him for years, that of his hypothetical electrotonic state. He was still convinced that it must exist and that he simply had not yet discovered the means for detecting it. Once again he tried to find signs of intermolecular strain in substances through which electrical lines of force passed, but again with no success.

It was at this time that a young Scot, William Thomson later Lord Kelvin , wrote Faraday that he had studied Faraday's papers on electricity and magnetism and that he, too, was convinced that some kind of strain must exist.

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He suggested that Faraday experiment with magnetic lines of force, since these could be produced at much greater strengths than could electrostatic ones. Faraday took the suggestion, passed a beam of plane-polarized light through the optical glass of high refractive index that he had developed in the s, and then turned on an electromagnet so that its lines of force ran parallel to the light ray.

This time he was rewarded with success. The plane of polarization was rotated, indicating a strain in the molecules of the glass. But Faraday again noted an unexpected result. When he changed the direction of the ray of light, the rotation remained in the same direction, a fact that Faraday correctly interpreted as meaning that the strain was not in the molecules of the glass but in the magnetic lines of force.

The direction of rotation of the plane of polarization depended solely upon the polarity of the lines of force; the glass served merely to detect the effect. This discovery confirmed Faraday's faith in the unity of forces, and he plunged onward, certain that all matter must exhibit some response to a magnetic field.


To his surprise he found that this was in fact so, but in a peculiar way. Some substances, such as iron, nickel, cobalt, and oxygen, lined up in a magnetic field so that the long axes of their crystalline or molecular structures were parallel to the lines of force; others lined up perpendicular to the lines of force.

Substances of the first class moved toward more intense magnetic fields; those of the second moved toward regions of less magnetic force. Faraday named the first group paramagnetics and the second diamagnetics. After further research he concluded that paramagnetics were bodies that conducted magnetic lines of force better than did the surrounding medium, whereas diamagnetics conducted them less well.

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