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		Modern Physics I
		Modern Physics II
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Last Modified: 2 February, 2009 Comments: Maiken Naylor

Modern Physics I

In the words of physicist Philip Morrison, "twentieth-century physics began about five years ahead of the century itself." The discovery of X-rays by Wilhelm Conrad Roentgen and Henri Becquerel's investigations of phosphorescent salts that fogged photographic plates whether or not they had been exposed to light ushered in the era of modern physics. Becquerel did not immediately realize that a previously unknown type of energy caused his so-called uranium rays - what Marie and Pierre Curie would call "radioactivity." Roentgen and the Curies appear on stamps from many countries around the world, whereas Bequerel did not capture the popular imagination, nor the attention of other than French postal authorities who decide whom to commemorate.

Wilhelm Conrad Roentgen (1845-1923) worked with cathode ray tubes and chemoluminescence. He discovered an invisible yet highly penetrating radiation which he called X-rays, and received the first Nobel prize in physics as a result. "Modern physics" had its beginning here. The year 1995, the centenary of his discovery, brought forth a spate of stamps from diverse countries, many of which show the striking skeletal image of Roentgen's wife's hand, with ring. Forever associated with radiology, and the medical applications of X-rays, Roentgen and his cathode ray tube have appeared on earlier issues as well.

Antoine Henri Becquerel (1852-1919) (Detail) was a French physicist whose research led him to the study of fluorescence in minerals. He discovered radioactivity when studying uranium compounds, which, unlike other fluorescent substances, fogged photographic plates even without having been exposed to strong light. He shared a Nobel prize for his discovery with Pierre and Marie Curie in 1903.

Marie Sklodowska Curie (1867-1934) was a double Nobel Prize winner, jointly with her husband Pierre (1859-1906) and Becquerel for the discovery of radiactivity in 1903, and by herself in 1911 for the discovery and isolation of radium. One of many stamps issued by her native Poland just emerging from the ravages of World War II dispenses with perforations. The Monaco stamp shows the cumbersome apparatus used to separate the raw ores from which radium was finally obtained. She is also shown with a glowing bowl containing her discovery, and again holding a laboratory sample. Together, the Curies appear on a stamp of Central Africa with what appears to be a very active molecular cluster. The word "radioactive" was first used by Marie Curie to describe her observations, as published in Comptes Rendus. Pierre Curie, (Detail) besides his work in radioactivity, was the discoverer of piezoelectricity. He also observed that permanent magnets lose their magnetic properties when heated above a critical temperature, which is called the Curie temperature.

Irene (1897-1956) and Frederic Joliot-Curie (1900-1958), daughter and son-in-law of the Curies, continued the study of radioactivity, producing artificial radioisotopes by alpha bombardment of aluminum and other light nuclei, and also shared a Nobel Prize in chemistry for this work. Frederic continued research on uranium fission and the accompanying production of neutrons. He became the leader of the French atomic energy program. After World War II, he had a deep concern for world peace, becoming the first president of the World Council for Peace.

Hendrik Antoon Lorentz (1853-1928)and his student Pieter Zeeman (1865-1943) shared the 1902 Nobel prize in physics for the investigation and explanation of what is now called the Zeeman effect: when a spectral source is placed in a magnetic field, the emission lines are found to be first broadened and then split into components as the magnetic field strength increases; moreover, the emitted light is polarized. Lorentz' theoretical work on the electromagnetic theory of light assumed that charged particles called electrons carry currents or transport electric charge, and their vibrations are the cause of electromagnetic waves. This preceded the identification of the electron, for which J. J. Thomson received the 1906 Nobel prize. The effect of a magnetic field on the light emitted and spectroscopically analyzed conformed to Lorentz' theory, confirming the electromagnetic wave nature of light. Further mathematical achievements by Lorentz include the Lorentz transformations, which describe the motion of bodies at close to relativistic speeds, and appear in Einstein's special theory of relativity.

Johannes Stark (1874-1957) was able to demonstrate the splitting of atomic spectral lines due to an electric field, just as Zeeman had done with a magnetic field. However, the patterns are not symmetrical about the original line, as in the Zeeman effect, nor are the relationships between field strength and change in wave length quite as simple, making the Stark effect not a useful tool for spectral analysis. Stark won the Nobel prize in physics in 1919 for this discovery, as well as for the observation of the Doppler effect in radiation emitted by accelerated hydrogen atoms in a discharge tube.

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