AtomicSpectra - Atomic Spectroscopy Johannes could still...

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Atomic Spectroscopy Johannes could still remember when he saw his first rainbow. It was at his uncle’s farm just north of his hometown of Halmstad, Sweden. As he studied the arch of brilliant colors that stretched over the fields of grazing cattle on that misty morning, he wondered how such a thing of beauty could exist. Now he was a professor at the prestigious University of Lund, and he was still studying rainbows. But in the laboratory, Johannes Rydberg was looking at a very different rainbow than what he had witnessed as a boy. He was looking at the light given off from a tube filled with hydrogen gas, and he observed this light as it passed through a prism and was refracted , or bent. In 1855, Heinrich Geissler invented a gas discharge tube that was similar to the tubes used in today’s “neon” signs. A glass tube is evacuated under vacuum, then gas is pumped in at low pressure. Electrodes are placed at the ends of the tube. When an electrical voltage is applied, the gas in the tube glows with a characteristic color. In the late 1800’s, gas discharge tubes were manufactured solely for the purpose of research so that people like J. R. Rydberg and J. J. Balmer could study their physical phenomena. When the light from these tubes is refracted, the result is a line spectrum , a series of thin lines of different colors that are separated by darkness. In contrast, when white light, such as that given off by the sun or an incandescent bulb, is passed through a medium that refracts the beam, the human eye sees a continuous spectrum of colors, like a rainbow. (See Tro, pp 281-306.) What is happening inside a gas discharge tube? The energy supplied by the voltage causes the electrons in the atoms of the gas to become “excited”; that is, they jump to a higher energy level. The excited electrons then fall to a lower energy level, releasing a photon of energy. Light can be related to energy by the following expression:
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AtomicSpectra - Atomic Spectroscopy Johannes could still...

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