Question

Passage:
In the early nineteenth century, optical instrument maker Joseph von Fraunhofer revolutionized observational astronomy by refining the manufacturing process of flint glass. Prior to Fraunhofer's innovations, optical telescopes suffered severely from chromatic aberration—the failure of a lens to focus all colors of light to the same convergence point—because available glass melted with inconsistent density and homogeneity. By developing specialized stirring techniques during the cooling phase of molten glass, Fraunhofer achieved unprecedented uniformity in large disk castings, allowing him to produce achromatic doublet lenses of exceptional clarity. In 1814, while testing the refractive properties of these superior flint glass prisms using a precision total-station darkroom apparatus, Fraunhofer observed hundreds of dark, narrow lines spanning the solar spectrum. Although previous observers such as William Hyde Wollaston had noted dark gaps in solar light, Wollaston had mischaracterized them as natural boundaries between discrete color bands. Fraunhofer systematically mapped over 570 of these lines, designating the prominent ones with capital letters from A to K. More importantly, he established that these dark lines appeared at fixed spectral positions regardless of atmospheric conditions or lens geometry. While Fraunhofer did not deduce the precise physical mechanism of atomic absorption—a breakthrough achieved decades later by Gustav Kirchhoff and Robert Bunsen—he explicitly demonstrated that these spectral lines were intrinsic features of sunlight itself rather than optical artifacts generated by the telescope lens or atmospheric interference.

According to the passage, Fraunhofer established which of the following regarding the dark spectral lines he mapped?