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Modern Physics III
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Albert Einstein's (1879-1955) name is associated
with the general and special theories of relativity, but his explanation
of the photoelectric effect, a phenomenon which could not be be accounted
for by electromagnetic wave theory, won him the Nobel prize in physics.
He proposed a corpuscular model of radiation as the photoelectric mechanism.
Light travels in quanta (or photons) and must be of a certain threshhold
energy (or color) to cause the emission of electrons from a given surface,
while its intensity determines only the number of electrons so released.
This German stamp shows electrons escaping at random angles while light
strikes the surface.
Einstein's law E=mc2 is shown on Nicaragua Scott 879 and features a stylized mushroom cloud generally associated with a nuclear bomb explosion. Inside the mushroom cap appears a beryllium atom with four electrons orbiting a nucleus of four protons and five neutrons. "Beryllium is a highly efficient generator of neutrons when bombarded with alpha particles. It may serve as a source of neutrons to initiate the nuclear fission within the fuel nucleus of a reactor." (Gmelin Handbook Be Suppl.vol. A1 p.124). At the base of the mushroom are roiling, churning clouds of debris as seen on actual photos of nuclear events. However, front and center in the picture is a prone white-draped figure being irradiated from upper left, watched by a technician behind a window to the right. This peaceful application of nuclear energy somewhat mitigates the stark symbolism of the mushroom cloud. The image of Einstein transcends national boundaries; here are but a few impressions of his somber countenance. The 14th International Conference on General Relativity and Gravitation was held in Florence in 1995 and was commemorated with this stamp. It shows the dome of Florence, Galileo, a pensive Einstein, and the general relativity field equation in its condensed form against a sky filled with constellations, among them Omega. G is the curvature of four-dimensional space-time, and T, a measure of the distribution and flow of energy and momentum, which is linked to the mass distribution. Gravity is no force but an aspect of the geometry of space-time. Space is not an absolute invariant entity, but is influenced by the distribution of mass and energy in the Universe. The basic field equation of general relativity therefore relates the curvature of space to the mass/energy density: G = k T. Galileo, who flourished in Florence, was aware of the principle of equivalence, central to Einstein's theory. |
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The word laser is an acronym for the phrase "light amplification by stimulated emission of radiation". This is a process already predicted by Einstein in 1917, in which an atom in an excited energy state, when struck by a photon of the correct size (corresponding to the energy difference between the excited state and a lower or ground state) will emit just such a photon, which will travel in tandem with the original impinging photon. Now there are two photons where there was one before, and in effect, the light has been amplified. Before the first laser was built by the physicist Theodore Maimam in 1960, the process was already used in the construction of the maser by Harold Lyons in 1949, amplifiying the natural vibrational frequency of the ammonia molecule, which lies in the microwave region of the spectrum, hence "m"aser. Since then, lasers have been built in many regions of the spectrum, of solid and gaseous materials, emitting continuous or pulsed radiation, which is monochromatic (since only one frequency is being excited) and can be collimated. High precision and intensity can be achieved by such beams, for example in surgery of the eye, as is pictured on the French stamp at left. Naturally occurring lasers have been observed in distant galaxies in the infrared, ultraviolet, and microwave regions. |
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While the invention of holography preceded the development of the laser, the technique never really came into its own until lasers were used to produce the coherent, monochromatic light necessary to create these novel representations of three-dimensional reality. While the light used in an ordinary photographic exposure is incoherent and registered only in its intensity on film, the reflection of coherent waves from different areas of an object preserves their phase and causes interference patterns that vary in intensity depending on the angle of observation. The Hungarian physicist Dennis Gabor developed the idea to let the light reflected from an object fall on a plate together with a light beam that had not been disturbed; the interference of these waves would recall the topography of the object. For this theory he received the 1971 Nobel prize in physics. The potential of holography in the examination of three-dimensional objects, including plasmas, is great, and holograms are can now be commonly seen on credit cards, and even postage stamps. This very beautiful British stamp shows a hologram of an atomic model of unknown species. Another stamp with a hologram is the German stamp showing the impact of comet fragments on Jupiter. |
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This German stamp commemorates two Nobel laureates, friends for life, and expatriates. (Detail) Max Born's work in quantum mechanics, at a time when both Heisenberg and Schroedinger developed their formalisms, linked wave mechanics with probability by interpreting the square of the wave function as a probability density. His contributions were recognized with the Nobel prize in physics in 1954. James Franck received his prize in 1926, for the famous Franck-Hertz experiment, which is a photoelectric effect in reverse. Together with Gustav Hertz, nephew of Heinrich Hertz, Franck bombarded mercury vapor with electrons and found that the electron energy dropped sharply at a certain level. The atoms absorbed the kinetic energy of the electrons, emitting it then in the form of light as resonance radiation. This confirmed the existence of discrete energy levels in atoms according to Bohr's theory of the atom and validated Planck's quantum theory. Gustav Hertz' later work was in isotope separation by diffusion cascade, used to produce enriched uranium, U235, among others. |
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Victor de Broglie (1892-1987) demonstrated mathematically that elemenary
particles should exhibit wave-like characteristics, just as under certain
circumstances light waves assume the properties of particles. The relationship
between mass, velocity, and wavelength of such a particle is shown on
the French and Nicaraguan stamps. A diffraction pattern and electron microscope
complete the picture on the latter. The wave-like behavior of electrons
was subsequently demonstrated by Germer and Davisson in 1927, and de Broglie
received the 1929 Nobel prize in physics. The Swedish stamp honoring de
Broglie's achievement suggests the wave-particle duality by actual electron
diffraction patterns in the shaded areas of the amplitude where the particle
associated with the wave is more likely to be. |
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 The Austrian physicist Erwin Schroedinger (1887-1961) (Detail)
incorporated the wave nature of the electron suggested by de Broglie into
the theory of the hydrogen atom, substituting standing waves to describe
the likely whereabouts of the electron in favor of the Bohr-Sommerfeld
orbits. These solutions to the Schrodinger wave equation are discrete
in that they depend on an integral number of wavelengths to occur in a
possible orbit so as not to cancel each other out. Two such electronic
states, in which the hydrogen atom is in a stable position, are shown
on the Swedish stamp. |
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