In 1801, the physicist Thomas Young clearly demonstrated that light travels as a wave, much like the succession of crests and troughs along the surface of water. The concept was not new – the Dutch physicist Christiaan Huygens had already proposed this idea during Newton’s time – yet Young was able to convince the scientific community by studying the patterns made by a beam of light as it passed through slits.

One of Young’s most astonishing feats was that he succeeded in measuring the wavelengths of red and violet light; that is, the distance between two crests or troughs of successive waves. His results revealed a wavelength of 7 thousandths of a millimetre for red light and 4 thousandths of a millimetre for violet. Combining these measurements with the basic concepts of Newton’s work, he concluded that light with a longer wavelength is less deflected than light with a shorter wavelength.

During the same historic period, around 1802, the British chemist William Hyde Wollaston uncovered a surprising fact: the spectrum of sunlight is not continuous. In fact, numerous black lines are superimposed on the colours of its spectrum. Unfortunately, this revelation did not attract much attention and no ideas were advanced to explain their presence.

Josef von Fraunhofer.Several years later, in 1814, the German optician Josef von Fraunhofer noticed the same black lines in the spectrum of sunlight. Although he did not understand their significance, he nonetheless measured their positions and catalogued a total of 324.

In 1859, the German chemist Robert Wilhelm Bunsen (who improved and popularized the burner that bears his name) devised an original analytical technique: he inserted mineral salts into the flame of his burner and observed the colours produced by the gas emitted. He was thus able to determine whether a particular substance was present in a mineral by observing a colour that, according to him, characterized that substance.

Gustav Robert Kirchhoff, a German physician and friend of Bunsen, proposed the additional step of dispersing the light emitted by the gas by using a prism. In this way, they would be able to study its spectra. The two researchers thus made a major discovery: each chemical element generates a series of Robert Wilhelm Bunsen.spectral rays that uniquely characterize the substance, much like a fingerprint identifies a person.

The invention of tTheir new analytical technique, spectroscopy, led to the discovery of many new chemical elements in the years ahead.

Kirchhoff was interested in the spectrum of sunlight and realized that Fraunhofer’s black lines corresponded exactly to the bright rays emitted by specific chemical elements. He deduced that the white light produced by the hot surface of the Sun was in part absorbed by certain chemical elements present in its cooler atmosphere, which produces black lines in the spectrum.
ASTROLab of Mont-Mégantic National Park

© 2006 An original idea and a realization of the ASTROLab of Mont-Mégantic National Park

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