One of the most important developments leading to the exploration of the interior of the atom, and to the eventual overthrow of the classical theories of physics, was spectroscopy; the other was the discovery of the subatomic particles themselves.
In 1823 the British astronomer and chemist Sir John Frederick William Herschel suggested that a chemical substance might be identified by examining its spectrum—that is, the discrete wavelength pattern in which light from a gaseous substance is emitted. In the years that followed, the spectra of a great many substances were cataloged by two Germans, the chemist Robert Wilhelm Bunsen and the physicist Gustav Robert Kirchhoff. Helium was first discovered as a new element following the discovery of an unexplained spectral line in the sun's spectrum by the British astronomer Sir Joseph Norman Lockyer in 1868. From the standpoint of atomic theory, however, the most important contributions were made by the study of the spectra of simple atoms, such as hydrogen, which showed few spectral lines. . Chemical Analysis.
Discrete line spectra originate from gaseous substances where, in terms of modern knowledge, the electrons have been excited by heat or by bombardment with subatomic particles. In contrast, a heated solid has a continuous spectrum over the full visible range and into the infrared and ultraviolet regions. The total amount of energy emitted depends strongly on the temperature, as does the relative intensity of the different wavelength components. As a piece of iron is heated, for example, its radiation is first in the infrared spectrum and cannot be .n; it then extends into the visible spectrum where the glow shifts from red to white as the peak of its radiant spectrum shifts toward the middle of the visible range. Attempts to explain the radiation characteristics of solids, using the tools of theoretical physics available at the end of the 19th century, led to the prediction that at any given temperature the amount of radiation increased with frequency and without limit. This calculation, in which no error was found, was in disagreement with experiment and also led to an absurd conclusion: A body at a finite temperature could radiate an infinite amount of energy. This required a new way of thinking about radiation and, indirectly, about the atom. . Infrared Radiation; Ultraviolet Radiation.
The Breakdown of Classical Physics
By about 1880 physics was serene; most phenomena could be explained by Newtonian mechanics, Maxwell's electromagnetic theory, thermodynamics, and Boltzmann's statistical mechanics. Only a few problems, such as the determination of the properties of the ether and the explanation of the radiation spectra from solids and gases, appeared unsolved. These unexplained phenomena, however, formed the .ds of revolution, a revolution that was augmented by a series of remarkable discoveries within the last decade of the 19th century: the discovery of X rays by Wilhelm Conrad Roentgen of Germany in 1895; of the electron by Sir Joseph John Thomson of Great Britain in 1895; of radioactivity by Antoine Henri Becquerel of France in 1896; and of the photoelectric effect by Hertz, Wilhelm Hallwachs, and Philipp Eduard Anton Lenard of Germany during the period from 1887 to 1899 (. Photoelectric Cell). Coupled with the disturbing results of the Michelson-Morley experiments and the discovery of cathode rays, or electron stream, the experimental evidence in physics outstripped all available theories to explain it.
In 1823 the British astronomer and chemist Sir John Frederick William Herschel suggested that a chemical substance might be identified by examining its spectrum—that is, the discrete wavelength pattern in which light from a gaseous substance is emitted. In the years that followed, the spectra of a great many substances were cataloged by two Germans, the chemist Robert Wilhelm Bunsen and the physicist Gustav Robert Kirchhoff. Helium was first discovered as a new element following the discovery of an unexplained spectral line in the sun's spectrum by the British astronomer Sir Joseph Norman Lockyer in 1868. From the standpoint of atomic theory, however, the most important contributions were made by the study of the spectra of simple atoms, such as hydrogen, which showed few spectral lines. . Chemical Analysis.
Discrete line spectra originate from gaseous substances where, in terms of modern knowledge, the electrons have been excited by heat or by bombardment with subatomic particles. In contrast, a heated solid has a continuous spectrum over the full visible range and into the infrared and ultraviolet regions. The total amount of energy emitted depends strongly on the temperature, as does the relative intensity of the different wavelength components. As a piece of iron is heated, for example, its radiation is first in the infrared spectrum and cannot be .n; it then extends into the visible spectrum where the glow shifts from red to white as the peak of its radiant spectrum shifts toward the middle of the visible range. Attempts to explain the radiation characteristics of solids, using the tools of theoretical physics available at the end of the 19th century, led to the prediction that at any given temperature the amount of radiation increased with frequency and without limit. This calculation, in which no error was found, was in disagreement with experiment and also led to an absurd conclusion: A body at a finite temperature could radiate an infinite amount of energy. This required a new way of thinking about radiation and, indirectly, about the atom. . Infrared Radiation; Ultraviolet Radiation.
The Breakdown of Classical Physics
By about 1880 physics was serene; most phenomena could be explained by Newtonian mechanics, Maxwell's electromagnetic theory, thermodynamics, and Boltzmann's statistical mechanics. Only a few problems, such as the determination of the properties of the ether and the explanation of the radiation spectra from solids and gases, appeared unsolved. These unexplained phenomena, however, formed the .ds of revolution, a revolution that was augmented by a series of remarkable discoveries within the last decade of the 19th century: the discovery of X rays by Wilhelm Conrad Roentgen of Germany in 1895; of the electron by Sir Joseph John Thomson of Great Britain in 1895; of radioactivity by Antoine Henri Becquerel of France in 1896; and of the photoelectric effect by Hertz, Wilhelm Hallwachs, and Philipp Eduard Anton Lenard of Germany during the period from 1887 to 1899 (. Photoelectric Cell). Coupled with the disturbing results of the Michelson-Morley experiments and the discovery of cathode rays, or electron stream, the experimental evidence in physics outstripped all available theories to explain it.
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