1814

Birth of Spectroscopy

Isaac Newton (1643–1727), William Hyde Wollaston (1766–1828), Joseph von Fraunhofer (1787–1826)

In 1672, experiments by Isaac Newton showed that sunlight is not white or yellow but instead is composed of many colors of light that can be separated into a spectrum because they refract slightly differently when passing through an object, such as a prism. Newton’s experiments were widely repeated and expanded by others, including fellow English scientist William Hyde Wollaston, who, in 1802, was the first to observe that some parts of the Sun’s spectrum showed mysterious dark lines.

Scientists needed a tool, a method, for understanding these dark lines and exactly where they occurred in the solar spectrum. In 1814 the German optician Joseph von Fraunhofer developed that tool, called a spectroscope, which was a specially designed prism that could be used to measure the positions or wavelengths of the lines in an experimental technique known as spectroscopy. He observed more than five hundred dark lines in the solar spectrum with his spectroscope—astronomers still call these Fraunhofer lines. In 1821 he constructed a higher-resolution spectroscope using a diffraction grating instead of a prism, and founded stellar spectroscopy by discovering that bright stars such as Sirius also have spectral lines, and they are different from the Sun’s.

By the mid-nineteenth century, physicists and astronomers were able to reproduce these kinds of lines in the laboratory by filtering light through various gases, thus discovering that the lines are caused by different kinds of atomic elements absorbing different, very narrow and specific, wavelengths of light. Spectroscopy instantly became the primary way to measure the atomic and molecular composition of distant light sources such as the Sun, planetary atmospheres, stars, or nebulae without having to touch the object directly—all that was needed was a telescope and some kind of spectral-line measuring device, or spectrometer. Indeed, spectroscopy from ground- and space-based telescopes and from orbiting and landed space missions to other worlds continues to be an important part of modern astronomy and space exploration.

A series of high-resolution visible light spectra of the Sun showing Fraunhofer lines, from the McMath-Pierce Solar Facility at Kitt Peak National Observatory in Arizona. Wavelengths increase from left to right in each row, starting with violet at lower left and ending with red at upper right.