SPECTRUM AND SPECTROSCOPE (Part 2 of 3)

Study of the lines in various spectra has helped build the modern theory of matter. Soon after Bunsen and Kirchhoff developed the use of spectral lines as a means of chemical analysis, scientists thought that the various lines were given off by atoms vibrating at different rates under the stimulus of heat. They believed that the faster vibrations resulted in the shorter waves that caused lines to appear toward the violet end of the spectrum.

Rydberg, Johannes Robert (1854-1919), Swedish physicist; worked on the spectrum.

In 1885 Johann Jakob Balmer (1825-98) discovered through experimentation that the various rates of vibration in a mass of glowing hydrogen bore a simple mathematical relation to each other. This indicated that some one type of "mechanism" was at work at varying rates within the hydrogen atom, giving off the different wavelengths. Balmer could not guess what this "mechanism" might be, however. Then Johannes Robert Rydberg (1854-1919) introduced further information on this subject and developed a formula named for him that described many more observed relations; but he also did not know what it was within the atom that vibrated. Finally, the answer came in 1913 from Niels Bohr (1885-1962), the renowned Danish physicist.

Bohr's theory, built largely upon knowledge from the study of radioactivity, held that the hydrogen atom consisted of an electron revolving like a planet around a central nucleus, or "sun." Bohr believed further that as an atom absorbed energy by being heated, for example this orbit would enlarge by definite amounts, each enlargement representing the absorption of one quantum, or "packet," of energy. When energy was emitted, as in the form of light, the electron would fall by steps into inner orbits, and the frequency of the light would depend upon how many orbits were traversed. If the electron fell inward by one orbit, the "energy splash" resulting from this would travel outward as light of a certain frequency. If it fell inward by two orbits, light of a different frequency would go forth. The collection of lines given by hydrogen in a spectroscope sums up these actions taking place in all the hydrogen atoms present. Furthermore, by using the Planck constant (the fundamental measurement of a quantum) and electrical factors in a formula of the Rydberg type, Bohr was able to reduce his whole explanation to terms of electrical force. Thus the spectrum of hydrogen was explained as the product of electrical forces within the atom, and the spectroscope became useful for studying the structure of matter.

Another significant discovery was that X rays could be made to give spectra just as visible light did. This was done by causing a beam of X rays to fall upon a crystal. The short rays of the X rays were diffracted in a pattern that revealed the arrangement of atoms in the crystal.