The Discovery

It is difficult to single out who first discovered polarized light. Early humans could have noticed a peculiar smudge when looking at the sky in certain directions. Moreover, polarization has lots of quirks and was discovered many times in different contexts: even today it is the subject of much research. But the official story goes like this:

Viking tales (circa 700)

Did the Vikings beat Bartholinus by a thousand years? As described elsewhere in Polarization.com, they might have used the polarization of the sky for navigation. Even if they didn't, they did discover Iceland. That (not so icy at the time) land later became the main source of Iceland Spar (nowadays, a more accurate name would be Mexican Spar as the Iceland deposits have been depleted). Iceland Spar had a prime role in the modern discovery of polarization and continues to be a preferred material to split the polarization components of light. It is easy to conceive that a Viking, exhausted after some foray, would have taken some time to play with those transparent crystals and noticed how images were doubled (or was it the beer?).

Bartholinus sees double (1669)

Iceland Spar was involved in the official discovery of polarization. This naturally occurring transparent crystal (optical quality Calcite, CaCO3) separates an image into two displaced images when looked through in certain directions. In 1669, a Danish mathematician at the University of Copenhagen, Erasmus Bartholinus, not only saw double, but also performed some experiments and wrote a 60-page memoir about the results. This was the first scientific description of a polarization effect (the images are polarized perpendicular to each other), and for his efforts he may be considered the discoverer of this hidden property of light.

Huygens waves Newton (1672)

	Christian Huygens developed a pulse-wave theory of light that he published in 1690 in his famous optical book "Traite de la Lumiere", while Isaac Newton pushed a corpuscular theory of light in his not less influential book "Optics" (Opticks) (1704) (however, see Note 1). Although in the end both were correct (or wrong) as light has a dual personality (wave AND particle), Huygens was closest to the modern view. Yet, in trying to explain double refraction, Newton asks in Question 26 of Optics: "Have not the rays of light several sides, endued with several original properties?"
	In 1672 Huygens derived the double refraction property of the Iceland Spar from a geometric wave construction, extending the construction method he had employed to explain refraction. The "Huygens Principle" considers each point on a wavefront the source of spherical wavefronts that add up to build the propagating wavefront. Huygens realized that if the velocity of light varied with the direction the spheres would deform to ellipsoids and thus was able to explain the refraction law for crystals such as Iceland Spar.
Huygens experimented by passing light through two crystals and rotating one with respect to the other. For example, he noticed that for some directions the second crystal doesn't double the two images from the first crystal. This meant that each of the two beams were somehow different to ordinary light. Huygens interpretation was that the crystal impressed a peculiar disposition into each beam while Newton thought that the crystal separated particles with different "sides".

Young and restless (1801)

Thomas Young had amazing broad interests and talents, "a la Leonardo". From his discoveries in medicine and science, Helmholtz concluded: "His was one of the most profound minds that the world has ever seen." He deciphered the hieroglyphics from Egypt (before Champollion) and contributed to the Rosetta Stone (by the way, the iconoclast physicist Richard Feynman deciphered the Mayan code, but only after others had done it). Perhaps as a result of the preeminence of the corpuscular theory of light, the study of polarization didn't advance much for the next one hundred years.  In 1801 Young did the famous double-slit interference experiment. This seemed to definitely prove that light behaves like waves by showing that light plus light can result in darkness (destructive interference).  He used his theory to explain such things as Newton rings and the rainbow supernumerary arcs. However, the contemporary discoveries on polarization seemed to contradict the wave theory.  How could the ether, a fluid, transmit a wave with sides? (Sound in air cannot be polarized). He wrote that same year to Malus: "Your experiments show the insufficiency of  a theory which I have adopted, but they do not prove it false."  The breakthrough came a few years later when Young showed (independently from Fresnel) that polarization arises from the transverse wave nature of light.

As this is the year 1801, lets recall that Iceland Spar was also critical in the birth of crystal science. A French ex-priest, Rene-Just Hauy, was one day showing a piece of calcite to a friend when he dropped it. He was amazed and bewildered that all the small pieces had similar shape. He studied other crystals and in 1801 published a book stating that there are six basic crystal forms, thus founding the science of crystallography.

Malus on the road of Hell (1808)

"Beware of living during interesting times," says the Chinese adage. The young Etienne Louis Malus didn't loose his head during the French revolution nor during the Reign of Terror, but had to follow the Napoleon army in its invasion of Egypt. He participated in the campaigns in Palestine and Syria, where he contracted the plague that would finally kill him some years later. But he had time to make several important contributions to the understanding of polarization. His most crucial discovery came when he was playing with a crystal of Iceland Spar in his apartment at the Rue d'Enfer (literally, Street of Hell) in Paris. He looked at the reflections of the setting sun from a window of the Luxemburg Palace across the street and noticed how the intensity varied when he rotated the crystal (the image of the sun is partially polarized upon reflection). He followed this with some more experiments showing that the ability to polarize light was not restricted to very special crystals but could be present in reflections from any ordinary substance, transparent or opaque, except for polished metals. He came up with the Malus law that predicts the intensity of the light transmitted through a polarizer when the angle of the transmission changes (square law).

Brewster: everything is transparent except glass (1812)

Sir David Brewster, a prolific scientific writer (about 315 journal articles) and ex-minister in the Church of Scotland, studied optical physics subjects like dichroism and absorption spectra, as well as more popular subjects like stereo photography. Fittingly, his two best known contributions to optical science are the law describing the "polarizing angle" (polarising angle, at the time) and the invention of the kaleidoscope (he wrote a book on just that).

Although Malus had previously found that at certain angle of incidence, specific for each material, the reflection was completely polarized, he had the misfortune of choosing water and glass for precise measurements. Malus conclusion, quoted in Brewster's 1815 paper, was: "The polarising angle neither follows the order of the refractive powers, nor that of the dispersive forces. It is a property of bodies independent of the other modes of action which they exercise upon light" (Note 2). Malus big problem was that the surface properties of glasses at the time were different to those of the bulk, so he couldn't fit the glass results with those of water. Brewster repeated Malus experiments for many precious stones and other materials; he got frustrated, but finally, having found that glass was the only anomaly, discovered the problem and snatched this law for his namesake ("Brewster angle"). In his words: "The index of refraction is the tangent of the angle of polarisation" and " . . . when a ray is polarised by reflection, the reflected ray forms a right angle with the refracted ray."

Arago turns colors at Biot (1811)

The Frenchmen Francois Arago and Jean-Baptiste Biot didn't see eye-to-eye.  Arago discovered "interference colors" by placing a sheet of mica between a glass reflector and a calcite prism.  Imagine the impression that the striking colors must have made on him! He also noticed that the colors remained if the glass reflector was missing but the background was the sky: the blue sky must be polarized!  He also saw evidence of circular polarization by replacing the mica by a quartz crystal. He presented a paper to the Paris Academy in 1811 on his preliminary results.  But the next year Biot (a previous collaborator) jumped-the-gun on him by presenting two papers much more comprehensive, taking the field away from him (he is considered the discoverer of circular polarization).  Biot also studied the sky polarization and found that the rainbow was polarized (Brewster saw a beautiful confirmation of Descartes law and his polarizing angle on this fact).  

Arago took revenge on Biot by helping his protege, Fresnel, develop the wave theory of light, as Biot was a lifetime corpuscular. Arago didn't like Napoleon either and refused in 1852 to take the oath of allegiance.

Fresnel's polar disorder (1821)

"I have decided to remain a modest engineer . . . and even abandon physics . . . I now see it as a stupid plan troubling oneself to acquire a small bit of glory"  said once Fresnel  (he had been anticipated in some discoveries, again!).  Augustin Fresnel was perhaps the biggest discovery of Arago and Fresnel was lucky that Arago was there to encourage him.  In the end, in spite of living only to the age of 39, he acquired more than a small share of glory, as the index of any optics book shows.  A brilliant mathematician, he used the Huygens principle and integral analysis to develop a general wave diffraction theory (Note 3).  He also used a mechanical model of the ether to find general equations for the intensities reflected and refracted at a surface (which coincide with those derived from electromagnetic theory).

Fresnel 1816 memoir to the Academie explained diffraction in detail and with unprecedented predictive power, but polarization was not mentioned.  He was aware of the problem but did not have an explanation at the time.  So polarization remained for several more years a big hole in the wave theory of light.  The key problem was unpolarized light (of course!).  The symmetry of unpolarized light suggested to Fresnel that light could have a longitudinal component: then, how did it convert to transverse polarized waves?  Finally, around 1821 it occurred to him that unpolarized light was made of transverse waves changing direction very fast.  The transverse nature of light also explained Fresnel experimental results showing that rays perpendicularly polarized could not interfere.  A wave theory of polarized light was born!  (well, it was not accepted universally, as it seemed to imply a solid ether!).  Although Young arrived to the same conclusions at about the same time there was no priority dispute (for a change!) between them: each gave credit to the other.

To be continued . . .

Notes:

[1] Huygens theory is better described  as a "pulse" rather than "wave" theory, as he never considered concepts like wavelength or frequency.  He thought of light in terms of impulses transmitted from one "ether particle" to another.  Because these impulses are in the direction of the motion he could never find a physical basis for polarization.  On the other hand, Newton trying to explain his rings, provided the light particles with periodic "fits" that alternatively predispose light for easy refraction or reflection. He went on to measure their frequency!

Newton asks in Question 13 of Opticks: "Do not several sorts of Rays make vibrations of several bignesses, which according to their bignesses excite sensations of several colours, much after the manner that the vibrations of the air, according to their several bignesses excite sensations of several sounds? And particularly, do not the most refrangible rays excite the shortest vibrations for  making a sensation of deep violet, the least refrangible the largest for making a sensation of deep red, and the several intermediate sorts of rays, vibrations of several intermediate bignesses to make sensations of the several intermediate colours?"  Newton toyed with the concept of waves but finally discarded it because of the rectilinear propagation of light.

[2] Brewster says: "This premature generalisation . . . is perhaps equalled only by the mistake of Sir Isaac Newton, who pronounced the construction of an achromatic telescope to be incompatible with the known principles of optics."  However, on the rest of the paper (On the laws which regulate the polarisation of light by reflexion from transparent bodies, Phil. Trans. (1815)) Brewster praises Malus; and he is also the author on a biography of Newton.

[3] Ironically, another brilliant mathematician, Poisson, believed that the theory was wrong and pointed out that Fresnel's theory predicted a bright spot in the center of the shadow of a circular screen: an impossibility! Arago performed the experiment and did find the "impossible" bright spot, now known as the "Poisson spot".