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In 1852,Sir George Gabriel Stokes (1819-1903) published a paper entitled Composition and Resolution of Streams of Polarized Light from Multiple Sources by which he showed that the theory that the polarization behavior of light could be characterized in terms of physically observable characteristics.
In this landmark paper, Stokes found that any state of polarized light could be completely described by reference to four characteristics that are now known as the Stokes polarization parameters, or collectively as the Stokes vector. The first parameter, I, is a representation of the total intensity of the complete optical field (both polarized and unpolarized light). The remaining three parameters describe the polarization state of the complete optical field, i.e. in terms of linear and circular polarized light. Stokes became interested in polarization as a means of explaining the Fresnel-Arago interference laws, which used un-polarized light sources for their experiments. These laws were based on an “ether”- based assumption on the wave theory of light1, and all the experiments that attempted to prove these theories had been conducted without reference to polarization. Of course, Fresnel and his successors were never able to characterize the unpolarized light mathematically. Stokes succeeded where others failed mainly because he abandoned an amplitude-based approach and instead he proposed an experimental definition. He postulated that the intensity or amplitude of unpolarized light remains unchanged when a polarizer is rotated, or when a retarder of some retardance value is introduced into the optical field. Stokes also proved that his four parameters could be applied not only to unpolarized light, but also to partially polarized and completely polarized light. Although Stokes' paper transformed mathematical principles for modeling light, it was unfortunately forgotten for nearly a century.2
The importance of Stokes’ findings were finally brought to the attention of the scientific community by the Nobel laureate S. Chandrasekhar in 1947. Subramanyan Chandrasekhar studied the evolution and stability of starlight, and proposed the existence of black holes where stars "used" to be. He first published his work in 1947, after which the existence of black holes became widely accepted. But Chandrasekhar wasn't awarded the Nobel prize in physics for another 36 years, owing to lack of proof, namely: Since no light can escape from a black hole, how can you prove something - intensity of light - that can not be seen? At the time this was assumed to be the ultimate unanswerable question in physics. Chandrasekhar found an answer by employing the Stokes parameters. He proposed that you could detect the presence of a black hole by measuring the polarization shift that would naturally occur when other light (in this case, star light) passed by a black hole.
It took decades for scientists to provide direct evidence of black holes by using new instruments to measure the polarization of starlight. Jim Kemp was the first, using photoelastic modulators at the Pine Mountain Observatory, near Bend, Oregon. Jim Kemp’s discovery was made possible by the extreme sensitivity of the photoelastic modulator (PEM). Thus the birth of the PEM based polarimeter was a direct result of proving the existence of black holes.
1. 19th Century Ether Theory, Michael Jenssen
2. Polarized Light, pp. 31-32 (2nd Ed., 2003) by Dennis Goldstein
3. Modeling Polarimetric Imaging by DIRSIG, Jason Meyers
4. Remarks on the History Of Hydrological Optics by Kusiel S. Shifrin
5. Instrumentation for Astrophysical Spectropolarimetry
By Christoph U. Keller
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