& Tornado Alley
Sir Isaac Newton discovered in 1666 that the index of refraction of glass was different for different colors of light. This meant, he believed, that any glass lens would have chromatic aberration; i.e., there would be rainbow fringes of any image no matter how sharply focused. Since chromatic aberration seemed unavoidable in lens systems Newton invented the reflective telescope based upon mirrors. However about the time he was giving up on the possibility of achromatic lenses glass makers were creating new glasses by using lead compounds as ingredients. The only glass available up to that time was made from sand, lime and soda, what is now called crown glass. This was a formula that went back to the Phoenicians. The introduction of lead not only changed the refractive index of glass but reduced the differences in the refractive index for different colors of light. The leaded glasses were called flint glasses.
The degree of dispersion for a glass is defined as a ratio ν given by
where nD, nF and nC are the refractive indices at wavelenghts of 587.6 nm, 486.1 nm, and 656.3 nm, respectively. These are the Fraunhofer wavelengths. (The letters D, F and C are arbitrary labels for lines of the spectrum of elements that are used as standards.) The D-line is the characteristic yellow gold color of the sodium spectrum. C and F are lines in the hydrogen spectrum. The values of ν are generally in the range of 30 to 60. Chromatic aberration can be virtually eliminated by using multiple elements in a lens system with differing refractive indices.
Using leaded glasses Chester Moore Hall in 1733 designed an achromatic lens system. By 1758 J. Dollond had obtained a patent in the United Kingdom for an achromatic lens system and began their manufacture.
The locus of the high level optical research shifted to the continent. The researches of Joseph von Fraunhofer concerning the spectra of elements made the meaurement of the optical qualities of glasses precise. Fraunhofer and collaborators built the first optical glass factory near Munich. Later such factories were also built in France, Switzerland and England.
It was not until the late 19th century that optical theory and practice advanced significantly beyond Fraunhofer's work. It was E. Abbe moved the technology to a new level. Abbe founded the Zeiss plant in Jena where he set a chemist, Otto Schott, to work examining the influence of different elements on the optical characteristics of glasses. The use of boron and barium compounds led to several categories of glasses; i.e, the borosilicate crown glasses, the barium crown glasses and the barium flint glasses. Schott went on to found a company which produced special glasses of these types. Companies with similar lines were formed in the other major industrial countries; e.g., Bausch and Lomb in the U.S., Parra-Mantois in France and G.B. Chance in the U.K. It is notable that the Eastman Kodak company in the U.S. developed a glass which did not contain silicon.
|Indices of Refraction and the ν-Values for Various Types of Optical Glasses|
|Light Barium Crown||1.570||1.573||1.580||57|
M. Herzberger, "Optical Qualities of Glass," in R.K. Luneburg's Mathematical Theory of Optics, University of California Press, 1964, pp.411-431.
Frank L. Pedrotti and Leno S. Pedrotti, Introduction to Optics, Prentice-Hall, 1987.
HOME PAGE OF Thayer Watkins