In 1801 a building in Munich collapsed on top of a fourteen year old apprentice mirror maker. He was dug out of the rubble alive, and the rescue was watched by the Prince Elector of Bavaria, who gave the boy money on the spot. Joseph Fraunhofer used it to buy a glass working machine and books, and to get himself out of an apprenticeship in which his master had forbidden him to study.
Fourteen years later he had made the finest optical glass in the world, invented the modern spectroscope, discovered several hundred dark lines in the light of the Sun, and given astronomy the tool it uses to determine what stars are made of. He was not trying to do any of that. He was trying to build better lenses.
The Lens Problem
A simple lens bends blue light more than red, so the image of a star comes to a focus at slightly different places depending on colour and is fringed with colour. An achromatic lens corrects this by combining two glasses of different dispersion, crown and flint.
Making one that works required knowing the refractive index of each glass with precision, and measuring refractive index requires light of a single definite colour. In 1814, at the secularised monastery at Benediktbeuern where the optical works had been set up, Fraunhofer was hunting for such a light source.
He tried a lamp flame and found a bright orange line that was reliably in the same place, which was useful. Then he turned the instrument on sunlight to see whether the same line appeared, and found instead that the solar spectrum was crossed by dark gaps.
574 Lines
He had been half anticipated. In 1802 William Hyde Wollaston in London had noticed seven dark lines in a solar spectrum and concluded that they were natural boundaries between the colours, which is a reasonable guess and entirely wrong. He did not pursue it.
Fraunhofer’s optics were better and his approach was systematic. Using a narrow slit, a good prism and a small telescope to examine the spread out spectrum, he mapped 574 lines, measured their positions, and labelled the strongest with letters from A in the red to K in the violet. The notation is still in use: the D lines are the pair in the yellow, the H and K lines are the pair in the violet that dominate the spectra of most stars.
He noticed that the D lines fell exactly where the bright orange line from his lamp flame had been. He did not know why, and he said so.
He also checked other sources. Venus and the Moon showed the same pattern as the Sun, which makes sense since they shine by reflected sunlight. Sirius did not: it showed a different set of lines. That single observation contains the whole future of astrophysics, since it means the pattern is a property of the light source rather than of the atmosphere, the prism or the eye.
The Grating
Fraunhofer’s second instrumental contribution is less famous and arguably more important. He built diffraction gratings, first by winding fine wire on a frame and then by ruling parallel grooves in glass with a diamond point, eventually achieving several thousand lines to the inch.
A prism spreads light out by an amount that depends on the glass, so it gives relative positions. A grating spreads it by an amount that follows directly from the spacing of the grooves and simple geometry, which means it gives absolute wavelengths. Fraunhofer measured the wavelength of the D line to about a part in a thousand.
That turns spectroscopy from a comparison into a measurement. It is also the point where the wave theory of light stops being a philosophical position and becomes a working tool, since gratings only make sense if light has a wavelength at all. The long argument between Newton’s particles and Huygens’s waves was being settled in the same decade by Young and Fresnel, and Fraunhofer’s ruled glass was in the middle of it.
What the Lines Meant
The explanation arrived in 1859, thirty three years after Fraunhofer’s death, from Gustav Kirchhoff and Robert Bunsen at Heidelberg.
A hot dense object emits a continuous spectrum. A hot thin gas emits light at a set of sharp wavelengths characteristic of the element it is made of. And a cool thin gas in front of a continuous source absorbs at exactly those same wavelengths, leaving dark gaps in the continuum.
The dark lines in sunlight are therefore the fingerprints of the elements in the Sun’s cooler outer layers, printed on the light on its way out. The D lines coincide with the emission of sodium because there is sodium in the Sun, which is also why a pinch of salt in a flame glows the same orange.
The consequence is one of the sharpest reversals in the history of science. In 1835 Auguste Comte had offered the composition of the stars as his standing example of knowledge forever beyond human reach. Within twenty five years it was a laboratory exercise.
By 1868 the technique was strong enough to find something on the Sun that had never been seen on Earth. A yellow line that matched no known element was observed during an eclipse and named helium, and it was another twenty seven years before anyone isolated it from a rock.
The Instruments He Left
Fraunhofer ran the optical works as a business and kept his glass making methods secret, which is why nobody else could match his lenses for a generation and why some of his metallurgical notes died with him.
His telescope objectives were the best in existence. The 24 centimetre refractor he built for the observatory at Dorpat went to Wilhelm Struve, who used it for the first systematic survey of double stars. A Fraunhofer heliometer went to Königsberg, and with it, in 1838, Friedrich Bessel measured the parallax of 61 Cygni and produced the first reliable distance to a star.
That measurement had been attempted repeatedly for two centuries and had defeated everyone, including Hooke, who built a two hundred foot column in London partly to try it. What finally settled it was not a bigger instrument but a better made one, from a workshop in Bavaria.
Fraunhofer died in 1826 at thirty nine, of tuberculosis, an occupational disease of glassworkers who spend their lives breathing furnace fumes and metal oxide dust. His epitaph reads Approximavit sidera. He brought the stars closer.
The Barcode Since
Everything modern astronomy claims about the universe beyond the solar system comes through those lines.
- Composition. Which elements a star contains, and in what proportion, which is how stellar evolution and the origin of the elements were worked out
- Motion. Lines shifted towards the red or the blue give velocity along the line of sight. That is how the expansion of the universe was discovered, and how most exoplanets were found
- Conditions. The width and shape of a line encode temperature, pressure, magnetic field strength and rotation
All of it rests on a young glassmaker who wanted a light source of one definite colour so that he could measure the refractive index of a batch of flint glass, and who wrote down carefully what he saw instead of what he was looking for.
The Faces Behind the Instruments
Fraunhofer sat for a portrait late in his short life, and it is one of very few images of a working craftsman among the scientific portraits of the period, most of which show gentlemen with books. The distinction is visible in the paintings themselves, and it says a good deal about who was allowed to count as a scientist in 1820.
Portraying Science is a 400 page collection of those images, from oil paintings and tempera to engravings and early photographs, running chronologically from the sixteenth century to the nineteenth. Read alongside the science, the portraits become a record of how each generation wanted its own knowledge to be seen.
No products in the cart. 