Joseph von Fraunhofer
Joseph von Fraunhofer (March 6, 1787 – June 7, 1826) was a German optician who was the first to study and classify the dark lines in the light spectrum that bear his name. During his lifetime he was known as the maker of the finest refractor telescopes, and practically momopolized the market for quality instruments of that type. He also perfected the use of fine gratings for producing and analyzing spectra.
Biography
Fraunhofer was born in Straubling, Bavaria, the son of Franz Xaver Fraunhofer and Maria Anna Fröhlich. Fraunhofer's father was a craftsman producing fine decorative glass. By the time Fraunhofer was 11, he had lost both his parents, and he was apprenticed to Philipp Anton Weichelsberger, a mirror and decorative glass manufacturer. While Fraunhofer must have learned much about glassmaking during his apprenticeship, the environment was generally oppressive, and he was not permitted even to attend classes on Sunday that are reserved for apprentices.
An event occurred in 1801 that was to change Fraunhofer's life and the history of science. Weichelsberger's complex, which housed both the glass studio and living quarters, suffered a collapse, and Fraunhofer and Weichelsberger's wife were trapped the rubble. Mrs. Weichelsberger lost her life in the incident, but Fraunhofer, protected by a strong cross beam where he happened to be standing at the time, survived.
The near-death and rescue of the young Fraunhofer caught the attention of Maximilian IV Joseph, Prince Elector of Bavaria, who is said to have been involved in the rescue effort. Maximilian invited Fraunhofer to his castle, and was able to provide some financial support for him. Maximilian also pressured Weichelsberger to allow Fraunhofer to attend classes on Sunday, and put him under the care of Joseph von Utzschneider, an attorney with entrepreneurial leanings. Soon after Utzschneider met Fraunhofer, he partnered with Georg Reichenbach and J. Leibherr to open the Mathematical Mechanical Institute Reichenbach Utzschneider Liebherr, devoted to the manufacture of surveying instruments that required high-quality lenses for their manufacture.
From apprentice to manager
In 1804, Fraunhofer, with the help of a grant from Maximilian, tried to establish his own business, but could not make enough to support himself and was forced to return to his former employer. But in 1806, Utzschneider offered Fraunhofer a position at the Institute, which was now located at Benediktbeuern Abbey, formerly in the hands of the Benedictine order. Here he learned the art of glassmaking from Pierre Guinand, who had already perfected some of his own innovations.
From this point on, Faunhofer began to show his worth, and after three years was promoted to a managerial position. By 1809, Fraunhofer was offered a junior partnership in the firm, and was put in charge of its day-to-day operations, which two years later meant managing a staff of over 40 people. By 1812 Fraunhofer was producing quality refractor lenses with a diameter of 7 inches, a respectable size for a telescopic lens at that time.
Fraunhofer wanted to solve the problem of phantom coloration in the images of telescopes and other optical instruments. To accomplish this task, he decided to analyze the light of the sun, and the spectrum of different colors it produced when it passed through a glass prism. In order to examine this effect more accurately, he examined the spectrum of sunlight using a telescope, and thus invented the first spectroscope. He was surprised to find scattered throughout the spectrum, dark lines the position of which did not change. William Hyde Wollaston observed the same lines in 1802, but drew no strong conclusions from the phenomenon.
In 1814, Fraunhofer published the results of his research on these dark lines, which he found not only in the spectra of the sun, but also of the moon, stars and flames. He measured the index of refraction for the different lines, which because they always lay in the same place in the spectrum, were an indication of the manner in which the rays of a particular color were refracted. He used the results of his research to mitigate the effects of optical dispersion, which is the source of phantom coloration that detracts from clear optical images.
In 1821, Fraunhofer used a wire mesh to explore the dispersion effects of small slits, and found that his mesh produced a spectrum of the light he was observing. Using formulas derived by Fresnel, he was able to accurately compute the wavelength of light of a particular color. He later used lines etched on a plate of glass covered with gold leaf to produce the same effect. Such an arrangement is called a diffraction grating, and its properties were first examined by the American astronomer David Ritterhouse in 1786. Ritterhouse, however, did not draw any conclusions from his discovery. Fraunhofer, however, used his new arrangement to produce a clear spectrum and to measure the wavelength of the light of various colors.
In 1820 Fraunhofer was made a full partner of his firm and director of the institute. Due to the fine optical instruments he had developed, Bavaria overtook England as the centre of the optics industry. Even the likes of Michael Faraday were unable to produce glass that could rival Fraunhofer's.
His illustrious career eventually earned him an honorary doctorate from the University of Erlangen in 1822, and in the same year was appointed keeper of the museum for the Royal Academy of Sciences in Munich. In 1824, the king of Bavaria awarded him the Order of Civil Merit, became a noble, and made an honorary citizen of Munich. Like many glassmakers of his era who were poisoned by heavy metal vapors, Fraunhofer died young, in 1826 at the age of 39. His most valuable glassmaking recipes are thought to have gone to the grave with him.
Telescope lenses
Fraunhofer's firm outfitted many of the respectable observatories of Europe with telescopes. A refracting telescope of almost 10 inches diameter was purchased by the Russian government for its observatory at Dorpat. It is said that he had plans to execute a telescope lens of 18 inches.
Legacy
Fraunhofer invented the spectroscope, and discovered 574 dark lines appearing in the solar spectrum. He also came very close to discovery made 40 years later that linked the patterns formed by these dark lines to the chemical composition of the light source.
Fraunhofer invented the diffraction grating and in doing so transformed spectroscopy from a qualitative art to a quantitative science by demonstrating how one could measure the wavelength of light accurately. He found out that the spectra of Sirius and other first-magnitude stars differed from each other and from the sun, thus founding stellar spectroscopy.
Ultimately, however, his primary passion was still practical optics.
Although Fraunhofer investigated the dark lines in the spectrum for the practical purpose of improving optics, he was aware of some of the interesting theoretical implications of his discovery. It remained for others to reveal the impact of Fraunhofer's work. Much of quantum mechanics, which clarifies the relationship between particles and waves, is based on an analysis of spectral lines.
In the practical sphere, his work solved many of the problems that opticians had been grappling with regarding the chromatic distortion that plagued optical systems. Later designers of telescopes and microscopes built on his successes.
Fraunhofer lines
The Fraunhofer lines are a set of spectral lines named after Fraunhofer. The lines were originally observed as dark features (absorption lines) in the optical spectrum of the Sun.
In 1802, English chemist William Hyde Wollaston was the first person to note the appearance of a number of dark features in the solar spectrum. In 1814, Fraunhofer independently rediscovered the lines and began a systematic study and careful measurement of the wavelength of these features. In all, he mapped over 570 lines, and designated the principal features with the letters A through K, and weaker lines with other letters.
It was later discovered by Gustav Kirchhoff and Robert Bunsen that each chemical element was associated with a set of spectral lines, and deduced that the dark lines in the solar spectrum were caused by absorption by those elements in the upper layers of the sun. Some of the observed features are also caused by absorption in oxygen molecules in the Earth's atmosphere.
The major Fraunhofer lines, and the elements they are associated with, are shown in the following table:
| Designation | Element | Wavelength (nm) | Designation | Element | Wavelength (nm) | |
|---|---|---|---|---|---|---|
| y | O2 | 898.765 | c | Fe | 495.761 | |
| Z | O2 | 822.696 | F | H β | 486.134 | |
| A | O2 | 759.370 | d | Fe | 466.814 | |
| B | O2 | 686.719 | e | Fe | 438.355 | |
| C | H α | 656.281 | G' | H γ | 434.047 | |
| a | O2 | 627.661 | G | Fe | 430.790 | |
| D1 | Na | 589.592 | G | Ca | 430.774 | |
| D2 | Na | 588.995 | h | H δ | 410.175 | |
| D3 (or d) | He | 587.5618 | H | Ca+ | 396.847 | |
| e | Hg | 546.073 | K | Ca+ | 393.368 | |
| E2 | Fe | 527.039 | L | Fe | 382.044 | |
| b1 | Mg | 518.362 | N | Fe | 358.121 | |
| b2 | Mg | 517.270 | P | Ti+ | 336.112 | |
| b3 | Fe | 516.891 | T | Fe | 302.108 | |
| b4 | Fe | 516.751 | t | Ni | 299.444 | |
| b4 | Mg | 516.733 |
Given their well-defined wavelengths, Fraunhofer lines are often used to characterize the refractive index and dispersion properties of optical materials.[1]
See also
- Optics
- Spectroscopy
- Fraunhofer line
- Timeline of solar astronomy
- Balmer series
Notes
- ↑ The Fraunhofer C-, F-, G'-, and h- lines correspond to the alpha, beta, gamma and delta lines of the Balmer series of emission lines of the hydrogen atom. The D1 and D2 lines form the well-known "sodium doublet," the center wavelength of which (587.5618 nm) is given the designation letter "D." There is disagreement in the literature for some line designations; e.g., the Fraunhofer d-line may refer to the cyan iron line at 466.814 nm, or alternatively to the yellow helium line (also labeled D3) at 587.5618 nm. Similarly, there is ambiguity with reference to the e-line, since it can refer to the spectral lines of both iron (Fe) and mercury (Hg). To resolve ambiguities that arise in usage, ambiguous Fraunhofer line designations are preceded by the element with which they are associated (e.g., Mercury e-line and Helium d-line).
ReferencesISBN links support NWE through referral fees
- Aller, Lawrence H. (1991). Atoms, Stars and Nebulae, 3rd ed.. Cambridge University Press. ISBN 0-521-32512-9.
External links
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