# The Atom Essay Research Paper The AtomAP — страница 2

radiation but misled him. He had sealed a photographic plate in black paper, sprinkled a layer of uranium salt onto the paper and “exposed the whole thing to the sun for several hours.” When he developed the photographic plate “I saw the silhouette of the phosphorescent substance in black on the negative.” He mistakenly thought sunlight activated the effect, much as a cathode ray releases Rontgen’s X rays from the glass. The story of Becqueerel’s subsequent serendipity is famous. When he tried to repeat his experiment on Feb. 26 and again on February 27 Paris was covered with clouds. He put the uncovered photographic plate away in a dark drawer, with the uranium salt in place. On March 1 he decided to go ahead and develop the play, “expecting to find the images very feeble. On the contrary, the silhouettes appeared with great intensity. I thought a t once that the action might be able to go on in the dark.” Energetic, penetrating radiation from inert matter unstimulated by rays or light: now Rutherford had his subject, as Marie and Pierre Curie, looking for the pure element that radiated, had their backbreaking work. But no one understood what produced the lines. At best, mathematicians and spectroscopists who liked to play with wavelength numbers were able to find beautiful harmonic regularities among sets of spectral lines. Johann Balmer, a nineteenth-century Swiss mathematical physicist, identified in 1885 one of the most basic harmonies, a formula for calculating the wavelengths of the spectral lines of hydrogen. these collectively called the Balmer series. It is not necessary to understand mathematics to appreciate the simplicity of the formula Balmer derived that predicts a line’s location on spectral bad to an accuracy of within on part in a thousand, a formula that has only on arbitrary number: lambdda=3646(n^2/n^2-4). Using this formula, Balmer was able to predict the wavelengths of lines to be expected for parts of the hydrogen spectrum not yet studied./ They were found where he said they would be. Bohr would have known these formula and numbers from undergraduate physics especially since Christensen was an admirer of Rydberg and had thoroughly studied his work. But spectroscopy was far from Bohr’s field and he presumably had forgotten them. He sought out his old friend and classmate, Hans Hansen, a physicists and student of spectroscopy just returned from Gottigen. Hansen reviewed the regularity of line spectra with him. Bohr looked up the numbers. “As soon as I saw Balmer’s formula,” he said afterward, “the whole thing was immediately clear to me.” What was immediately clear was the relationship between his orbiting electrons and the lines of spectral light. Bohr proposed that an electron bound to a nucleus normally occupies a stable, basic orbit called a ground state. Add energy to the atom, heat it for example, the electron responds by jumping to a higher orbit, one of the more energetic stationary states farther away from the nucleus. Add more energy and the electron continues jumping to higher orbits. Cease adding energy-leaving the atom alone-and the electron jump back to their ground states. With each jump, each electron emits a photon of characteristic energy. The jumps, and so the photon energies , are limited by Plank’s constant. Subtract the value of a lower-energy stationary state W2 from the value of a higher energy stationary state W1 and you can get exactly the energy of light as hv. So here was the physical mechanisms of Plank’s cavity radiation. From this elegant simplification, W1-W2=hv, Bohr was able to derive the Balmer series. The lines of the Balmer series turn out to be exactly the energies of the photons that the hydrogen electron emits when it jumps down from orbit to orbit to its ground state. Then, sensationally, with the simple formula, R=2pi^2me^4/h^3, Bolar produced Rydberg’s constant, calculation it within 7 percent of its experimentally measured value. “There is nothing in the world which impresses a physicist more,” an American physicist comments, “than a numerical agreement between experiment and theory, and I do not think that there can ever have been a numerical agreement more impressive than this one, as I can testify who remember its advent.” “On the constitution of atoms and molecules” was seminally important to physics. Bexzides proposing a useful model for the atom, it demonstrated that events ensts that take place on the atomic scale are quantized: that just as matter exits as atoms and particle s in a state of essential graininess, so also does process. Process is discontinuous and the “granule” of mechanistic physics was therefore

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