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Modern Physics II
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The Austrian physicist Ludwig Boltzmann (1844-1906)(Detail) developed and advanced the theories of thermodynamics, the kinetic theory of gases, and electromagnetism at a time when new discoveries about the structure of matter were being made. Boltzmann supposed that the behavior of gases depended on the movement of their atoms or molecules. From a consideration of the second law of thermodynamics he arrived at a mathematical formulation of the fact that equilibrium is the most probable state for a gas to achieve. As expressed by his famous equation, entropy is proportional to the log of probability: the Boltzmann constant, k, is named after him. The Nicaraguan stamp (Detail) features an internal combustion engine as an application of kinetic theory. Boltzmann is also commemorated on the Austrian stamp bearing his likeness.. |
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Max Planck (1858-1947) solved the dilemma of describing accurately the frequency distribution of radiation from a black body, a problem that had been approached from both the high (Wien) and low (Rayleigh) frequency ends, but with no apparent smooth overlap. Planck postulated that electromagnetic energy was not infinitely divisible, but existed in particles, or quanta, proportional to the frequency of the radiation. The Planck constant, h, in the exponent of the black body spectral distribution equation tamed the ultraviolet catastrophy to produce distribution curves such as shown in the Fraunhofer stamp of radiation from the sun. Planck received the 1918 Nobel prize in physics, and Einstein and Bohr applied quantum theory to the photoelectric effect and atomic structure, explaining away some baffling paradoxes of classical physics. The German stamp shows a black body emitting a quantized amount of energy, while the spectrum of this radiation appears continuous to the eye. Planck's constant also commemorates the anniversary of his death in 1947. |
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 Ernest Rutherford (1871-1937)(Detail) worked with radioactivity, which he found to be of
different kinds, distinguishing between positive, or alpha rays, and negative,
or beta rays. In this work he was assisted by the German physicist Hans
Geiger ( ), with whom he devised an instrument to detect and count these
particles. The instrument consisted of a tube containing gas with a wire
at high voltage along the axis. Each particle entering the tube ionized
some gas, causing an electron avalance and a measurable pulse, or click.
He discovered that the alpha particles were helium nuclei. By 1909, Geiger
and E Marsden used alpha particles to bombard very thin metal foils. Sometimes
the alpha particles were scattered backward as if repelled by other positive
charges; this led Rutherford to propose the theory of a nuclear atom,
containing a very small positive nucleus, surrounded by negatively charged
electrons and a great deal of empty space. In 1928 Geiger and W. Muller
designed an improved counter to measure radioactivity, the Geiger- Muller
counter. |
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 Danish physicist Niels Bohr (1885-1962) (Detail) won the Nobel prize in physics for his model of the
hydrogen atom, in which the electron occupied discrete energy levels,
or orbits, around the nucleus, and radiated or absorbed energy only when
moving between energy levels, whereas classical theory predicted that
an orbiting electron (a moving charge) should radiate energy continuously.
This model of circular orbits was able to account for the major series
of the hydrogen spectrum, but it was left to Sommerfeld to explain its
fine structure by adding elliptic orbits to the model, resulting in the
Bohr- Sommerfeld atom. Bohr is commemorated with circular electron orbits
on the Swedish stamp but with an elliptic orbit on the Danish stamp, which
also indicates the quantized nature of the energy transition between orbits.
The stamp showing Bohr and his wife on a garden bench enjoys some minor
fame because the bench only has three legs. |
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 Max von Laue (1879-1960) (Detail) postulated that diffraction of X-rays in a crystal
might occur if lattice spacing and X-ray wavelength were of the same order
of magnitude. Patterns such as those on the stamps were obtained experimentally,
proof of the periodic structure of crystals and the wave nature of X-rays,
and sufficient to win him the 1914 Nobel physics prize only two years
after successful experiments. |
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Sir William Henry Bragg (1862-1942) and Sir William Lawrence Bragg (1890-1971), father and son, were intensely interested in the X-ray diffraction work done by von Laue. They used his discovery to study crystal structure and developed the X-ray spectrometer. Their Nobel prize in physics followed von Laue's by a year, in 1915. The Royal Institute of Chemistry stamp shows the crystal structure of salt, as determined by WLB, and used by WHB to determine X-ray wavelengths. |
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