Sunlight, spectral lines

It often takes time for the implications of experimental data to be understood and to be acted upon. Fraunhofer s earlier observation that the solar D-lines coincided with the spectral lines of a sodium lamp eventually prompted further important experiments. In 1849, Jean Bernard Leon Foucault (1819-1868), a Parisian physicist, made an unexpected discovery. He passed sunlight through a vapor of sodium and he found that the solar D-lines were darker. His conclusion was that the sodium vapor presents us with a medium which emits the rays D on its own account, and which absorbs them when they come from another quarter. The consequences of Foucault s experiment, however, were expressed more cogently by Sir William Thomson (later Lord Kelvin). He drew the following explicit conclusion That the double line D, whether bright or dark, is due to the vapor of sodium. . . That Fraunhofer s double dark line D, of solar and stellar spectra, is due to the presence of vapor of sodium in atmospheres surrounding the Sun and those stars in whose spectra it has been observed. ... 

One of the earliest and most famous examples of the use of spectrometry to study the solar system occurred in 1868. While observing a solar eclipse in India, the French astronomer Pierre Janssen (1824-1907) found a new spectral line in sunlight very close to one found in the spectrum of sodium. Janssen was able to show, however, that the new yellow light was different from the sodium line. Indeed, it was a spectral line that had never before been observed on Earth. Janssen hypothesized that the presence of the line could be explained only if the Sun s atmosphere contained an element that had not yet been discovered on Earth. The British astronomer Sir Norman Lockyer (1836-1920) later suggested the name helium for the element, a name based on the Greek word for "sun," helios. It was almost 30 years later that Janssen s bold hypothesis was confirmed. In 1895, the British chemist and physicist Sir William Ramsay (1852-1916) first detected helium on Earth, during a series of experiments on an ore of uranium called clevite. 

Helium is present in very small quantities in the atmosphere. Its presence in the sun is shown by the occurrence of its spectral lines in sunlight. These lines were observed in 1868, long before the element was discovered on earth, and the lines were ascribed to a new element, which was named helium by Sir Norman Lockyer (1836-1920),... 

The arc lamp with the best resemblance to sunlight is the xenon arc lamp, although the development of new metal-halide lamps has led to competition for that claim. The spectral outputs of these two types are shown in Figure 3.2 and Figure 3.3. The xenon arc has a relatively smooth continuous output spectrum with some line emissions superimposed in the region of 450 to 500 nm, whereas the metal-halide lamp is more uniform across the 350 to 550 nm region. 

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