This is my entry for the fifth edition of The Giant’s Shoulders, to be held at Podblack Blog on November 15th.
By the late 1800s, physicists had begun a serious study of the structure of the atom. The best tool for such studies, indeed pretty much the only tool in that pre-quantum era, was a spectroscopic analysis of the light emitted/absorbed by the atom. Each species of atom radiates light at its own distinct, discrete set of frequencies (‘spectral lines’), and knowledge of these frequencies could be used, for instance, by astronomers to determine the chemical composition of distant stars.
Devices which could be used to analyze the spectral content of a light field in that era were, however, limited. Like a far-sighted person trying unaided to read the fine print of a newspaper, the spectroscopes of the time were limited in how well they could distinguish (‘resolve’) very closely-spaced spectral lines. Researchers needed a device which could outperform existing techniques, such as the use of a diffraction grating or a Michelson interferometer.
In 1897, Charles Fabry and Alfred Perot introduced a new interferometric device which would eventually bear their name: the Fabry-Perot interferometer. The design of the interferometer is, in principle, simplicity itself: light is passed through a pair of parallel, highly reflecting mirrors. Interference between components of the light undergoing multiple reflections result in extremely well-defined interference fringes emerging from the device, from which spectral properties of light can be deduced.
Fabry and Perot published a large number of papers on their interferometer, including 15 joint articles between 1896 and 1902. The first of these articles dealt with the theory of the interferometer and applied it to the accurate measurement of very small distances, while the second described the apparatus in detail and applied it to spectroscopy.
In this post, we discuss the scientists, their interferometer, the results of their first few papers, and the impact the F-P interferometer has had on physics in general.
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