Devices to Demonstrate Polarization Phenomena
Thomas B. Greenslade, Jr.
Gambier, Ohio 43022 USA
The discovery of polarization effects in the early years of the 19th century led to the development of a wide range of optical instruments to demonstrate the phenomena of polarization. The surviving examples show ingenious ways to show polarization effects before all were rendered obsolete by the invention of the polarizing filter by Land in the 1920s.
Discoveries up to the Early 19th c
The first twenty five years of the nineteenth century saw a flurry of research activity in the study of polarized light, which had been discovered over one hundred years earlier by Bartholinus and Huygens. During this short period, Biot, Seebeck, Wollaston, Brewster, Arago, Malus, Herschel and Fresnel discovered and interpreted almost all of the phenomena of polarization, and a number of instruments were devised to produce polarized light and study the effects of its passage through liquids and crystals.
A chronology of experimental discoveries and theoretical models helps to organize the rapid development of the field, and gives the background needed to examine the operation of the apparatus to be described.
1669. Erasmus Bartholinus (or Bartholin) (1625-98) first described double refraction in calcite, or Iceland spar. This now familiar phenomenon is shown in Fig. 1: 1 a natural crystal of calcite is laid over an object which appears to be doubled when viewed through the calcite. Bartholinus treated this effect as an unusual case of refraction. For a beam of light perpendicular to the surface of the crystal, one ray (the O or Ordinary ray) is transmitted without being refracted at the surface, while, most surprisingly, a second ray (the E or Extraordinary ray) has a non-zero angle of refraction.
1678. Christiaan Huygens (1629-95) extended Bartholinus’ discovery by examining the way in which the O and E rays produced by a calcite crystal were refracted as they entered a second calcite crystal. Today, we would say that Huygens had discovered that the E and O rays were polarized at right angles to each other. The two images of a dot produced by a calcite prism can be made to disappear in turn by rotating a polarizing filter before the eye. Huygens applied his longitudinal wave theory of light to the passage through calcite in his Treatise on Light, published in 1690.
1808. Etienne Malus (1775-1812) discovered polarization by reflection. Malus observed the light of the setting sun, reflected from the windows of the Luxembourg Palace in Paris, through a crystal of Iceland spar. As he rotated the crystal, the two images of the sun became alternately stronger and weaker, though there was never complete extinction. Almost at once, Malus repeated the experiment under controlled conditions, and found the angles at which complete extinction of the reflected ray was obtained for water and for a sample of glass. In 1811 Malus introduced the word “polarization”; he assumed that light consisted of particles which had poles which were lined up parallel to each other by the processes of reflection and refraction.
1814. David Brewster (1781-1868) discovered that light reflected from a surface is completely polarized when the angle of incidence, Θ, is related to the index of refraction, n, by the relationship n = tan Θ. This is now known as Brewster’s law, and the angle is called Brewster’s angle.
The model in Fig. 2 purports to illustrate polarization by reflection at two successive surfaces. The first surface shows the removal of one component of the incident light, and reflection from the second surface removes the remaining component. The geometry of this model is suspect, since, by Brewster’s law, the 45° angle of incidence is appropriate for an index of refraction of 1.000 — which means that there is no interface at which reflection can take place! The 1860 Ritchie catalogue has a very similar piece of demonstration apparatus at a price of $10.00, and this, along with illustrations in several textbooks of the period, has the impossible index of refraction for a reflecting surface.
The apparatus in Fig. 3 was purchased by the United States Military Academy in 1829, probably from Lerebours of Paris, and was used for the determination of Brewster’s angle for liquid or solid samples. A beam of unpolarized light is defined by passing through a hole in the bracket at the right end of the pivoted arm, and the reflected light is viewed through the eyepiece, which contains a calcite crystal. The calcite analyzer produces two images of the hole in the bracket, and at Brewster’s angle one of these disappears. A tourmaline crystal analyzer, or a Nicol prism (invented in 1828), would eliminate the extraneous image, but this is not really necessary for the use of the instrument. The reflecting surface is set parallel to and along the 90° line of the divided circles by the adjusting screws. The eyepiece can be moved outward to examine the light reflected from liquids in the square pan next to the solid surface. The apparatus is beautifully made of brass, but has a serious failure in human engineering. When I tried to use it during a visit to the National Museum of American History at the Smithsonian Institution, my nose hit the sharp-edged brass upright (I am right-eyed); when I shifted to the left eye, my forehead got in the way.
1813-16. The property which is now known as dichroism was discovered in agate by Brewster, and in tourmaline by Jean-Baptiste Biot (1774-1862) and Thomas Seebeck (1770-1831). Here the crystal is double refracting, as in the case of calcite, but one of the components is preferentially absorbed in the crystal. The availability of a single beam of linearly polarized light, instead of two beams polarized perpendicular to each other and separated by only a small transverse distance (as produced by calcite), made investigations with polarized light much less cumbersome and easy to interpret.
The tourmaline tongs in Fig. 4 have two thin slices of tourmaline mounted in rotating holders so that the transmission axes of the crystals may be either parallel or at 90° to each other, giving maximum or minimum transmission. The tongs are designed to allow the viewing of light transmitted by thin, transparent materials placed between the crossed polarizing filters. Most of the apparatus manufacturers sold small decorative designs (birds, flowers or geometrical designs) to be viewed through the tongs. The patterns were made of thin slices of mica or selenite (a crystalline variety of gypsum) mounted on a thin glass plate. The plate was clamped in the tongs and held to the eye while looking toward an evenly illuminated surface. As one of the tourmalines was rotated, the colors which were produced changed. These tongs are in the National Museum of American History at the Smithsonian Institution.
Nicol’s significant innovation and subsequent developments
1828. William Nicol (1768-1851) of Edinburgh developed what we now call the Nicol prism. The problem with calcite as a polarizer is the presence of two beams of polarized light. In principle, the extraordinary ray can be eliminated by using a narrow crystal, long enough so that the E ray can be sufficiently displaced from the O ray to allow it to be masked off. Nicol used the now classic method of slicing the crystal diagonally and cementing the two halves back together with a cement (Canada balsam) of such an index of refraction that the E ray is totally reflected at the internal interface, leaving the O ray to emerge alone from the crystal. The title to Nicol’s 1829 paper describing the prism, “On a Method of So Far Increasing the Divergency of the Two Rays in Calcareous Spar [calcite] That Only One Image May Be Seen at a Time” shows his general line of attack on the problem.
The Nicol prism by Duboscq in Fig. 5 is in the Garland Collection at Vanderbilt University. According to Robert Lagemann “its faces measure about 4.5 and 4.7 cm … it can be inserted into the Duboscq projector or the solar microscope and polarization of light can be demonstrated.” 2 The typical angle of the front and rear faces of 68° with respect to the long axis of the crystal can be seen. The Nicol prism is colorless, unlike the tourmaline polarizing crystal, but its basic long, narrow construction unfortunately gives it a small angular field of view. It was almost always used as an analyzer; only in the saccharimeter, the specialized instrument used to determine the degree of rotation of the plane of polarization by various liquids, is it used as a polarizer.
In 1814, Seebeck and also Brewster discovered that glass could be made doubly refracting when stressed by internal forces created by rapid cooling; the same effects are also produced when external forces are applied to the glass. This phenomenon has great practical importance in engineering today where transparent plastic models of complex structures are studied to see where stresses are concentrated. The Glass Press in Fig. 6, usually called a Fresnel Press, is used to strain the glass, which is placed between two crossed polarizing devices. The figure shows two presses, one for compressing square glass samples, and the other for bending long, slender ones. These were made by Soliel in the latter half of the 19th century, and are at the apparatus collection of Washington and Lee University.
At times it is necessary to have a wide field of view for specimens between crossed polarizing devices. Before the introduction of sheet polarizers, the practical solution to the problem of producing a linearly polarized beam of large diameter was the use of polarization by reflection. These instruments are usually called Polariscopes or Polarimeters and several examples will be discussed.
The form of Polariscope in Fig. 7 was described in 1873 by Edward C. Pickering (1846-1919), at that time at M.I.T; his subsequent career was at Harvard. Light illuminates the [missing] ground glass screen on the left, and is reflected from the horizontal glass plate at Brewster’s angle. The resulting plane-polarized light passes through the sample held in the holder on the angled bracket. The eyepiece contains a Nicol prism that acts as a second polarizer. In use, the transmission axes of the two polarizers are set at right angles to each other, and birefringent samples made of thin slices of mica mounted on glass are placed in the holder.
The Polariscope in Fig. 8 is an all-metal design that was sold by the Gaertner Scientific Corporation ca. 1930. The samples are contained in the hollow base that supports the mirror. The availability of the Polaroid™ polarizing filter in the 1930s made this instrument, with its expensive Nicol prism, redundant, and it no longer appears in the 1950 Cenco catalogue.
By comparison, the Biot polarimeter (Fig. 9), which is shown in the majority of the textbooks which discuss polarization phenomena, is awkward to use because it uses a second piece of dark glass, set at Brewster’s angle, as its analyzing filter. Either the eye, or the light source, or possibly both, are going to be in inconvenient locations to get the proper light path. The advantage of the Biot design, which dates from 1816, is the cheapness of the reflection polarizer compared to the expensive Nicol prism. This example is marked Duboscq/Soliel and is in the apparatus collection at Dartmouth College.
The Biot polarimeter in Fig. 10 is a rather attractive design that eliminates the crick-in-the-neck problem inherent in the apparatus in Fig. 9. Inside the turned wooden cylinder is a mirror set at Brewster’s angle. With the apparatus set at a convenient height on a table top, the user looks vertically into the hole in the cylinder. The sample is placed on top of the wooden box. This instrument seems to be one of a kind, and is in the collection of the American Museum of American History of the Smithsonian Institution.
An even less expensive polarimeter can be made using the optical arrangement in the upper left-hand corner of Fig. 11. 3 Light from a broad source located to the left is incident on a pile of glass plates at Brewster’s angle. The light which is reflected downward is completely plane polarized in a direction perpendicular to the plane of incidence (this is the plane containing the incident ray and the normal to the surface). The sample (assume that it is a thin sheet of mica), is placed on a mirror laid horizontally. The plane polarized light thus passes through the sample twice. When the light strikes the glass plates again, the transmitted light grows increasingly more polarized in the plane of incidence as more air-glass interfaces are reached. In making a mockup of this, I found that a stack of seven or eight plates suffices for ordinary purposes. 4 The effect is thus that of placing the sample between crossed polarizing filters. If it is desired to have an angle other than 90° between the transmission axes of the polarizer and the analyzer, a separate, hand-held stack of plates may be used, as shown in the remainder of the figure.
This arrangement, known as the Nörrenberg doubler, was developed in 1858 by Johann Gottlieb Christian Nörrenberg (1787-1862), professor of physics at Darmstadt and later at the University at Tübingen in Germany. Nörrenberg’s name has suffered over the years; in various nineteenth century texts it is spelled Nörremberg, Norrenberg and Narrenberg. The word “doubler” indicates that the light passes twice through the sample, thereby doubling its effective thickness. A quarter-wave plate thus becomes a half-wave plate, for example.
The Nörrenberg apparatus in Fig. 12 is in the Greenslade Collection, and was made by the Paris manufacturer Duboscq, who was well known for his optical instruments. A Nicol prism is used as an analyzer, but a small circular glass plate reflector [in front of the instrument] may be placed on the intermediate stage and used as an analyzer. In this format, the removable black iron shield keeps stray light from the source from the field of view in the glass plate.
Figure 13 shows an early 20th century Nörrenberg doubler. Light reflected by the black glass reflector at Brewster’s angle is polarized, passes through the sample held on the three-tine fork and is viewed through the Nicol prism in the eyepiece. The device was sold by Max Kohl of Chemnitz, Germany for 50 Marks (about $13) in 1900, and is in the Greenslade Collection.
The Saccharimeter (Fig. 14 below) is a specialized form of polarimeter that is primarily used to identify liquids by their ability to rotate the plane of polarization. Polarized light produced by the Nicol prism on the right-hand side of the apparatus travels through the empty glass cell, and a second Nicol prism on the left-hand side is rotated until the light (usually the yellow light from a sodium lamp) is extinguished. The liquid sample is placed in the cell, and the amount by which the Nicol prism has to be rotated to get extinction once more is noted. This angle is a linear function of the concentration of the sample. The name comes from its use in distinguishing between sugars which produce right-handed rotation of the plane of polarization (dextrose) and left-handed rotation (levulose or fructose). They were basic instruments in chemistry laboratories, and are not very often found in physics laboratories. The instrument in Fig. 14 is now in the Greenslade Collection, and dates from the early years of the 20th century. I have used it a number of times in upper-division optics laboratories.
- J. Dorman Steele, Fourteen Weeks in Natural Philosophy, (New York: A.S. Barnes & Co., 1869), p. 211. ↩
- Robert T. Lagemann, The Garland Collection of Classical Physics Apparatus at Vanderbilt University (Folio Publishers, Nashville, Tennessee, 1983) pg 189. ↩
- George M. Hopkins, Experimental Science, New York: Munn & Co. (1893), p. 245. ↩
- Thomas B. Greenslade, Jr., “An Inexpensive Modern Nörrenberg Doubler”, Phys. Teach., 19 (1981), pp. 626-627. ↩