POLARISED LIGHT

Te Awamutu Courier, Volume 52, Issue 3771, 19 June 1936, Page 3

POLARISED LIGHT

ITS HISTORICAL AND SCIENTIFIC ASPECTS My colleague (C.S.H.), who contributed an article on the recently invented polarising medium for light has suggested in discussion (writes K.G. in the Melbourne Age), that I should also write upon this topic, enlarging somewhat further upon its historical and scientific aspects. I may perhaps begin by offering a word of personal testimony to the genuineness and merit of this new aid to vision. Last year, when notices concerning Polaroid first began to appear in American scientific journals, I sent an order for two small samples, each about half an inch across. I have these beside me as I write and can testify that, while not possessing the 100 per cent, efficiency of first-rate Nicol prisms—of which more later—they certainly cut out a very high proportion of polarised light. Used as spectacles they certainly abolish “glare” most effectively. My colleague has given briefly some details of the story of his invention of Polarised glass by Mr Edward Land, of Harvard University. Behind this immediate story there lies a history of polarisation of light dating back to 1670, when Erasmus Bartholinus, a Dane, published an account of his observations on the properties of the crystalline substance Iceland spar—the name implies the place of origin which chemically, is nothing more or less than pure carbonate of lime and which is otherwise known to the mineralogist to-day as calcite.

Calcite crystals of good optical quality are as transparent as the clearest glass, and, if their dimensions exceed two or three inches, hard to obtain and very expensive. But none of the many hundreds of crystalline •substances now known to possess the same power of sorting out a beam of ordinary chaotically vibrating light into two separate beams in each of which the vibrations are confined to a particular plane has been found superior to calcite, as a polarising medium. UNEQUAL REFRACTION These two beams, as Bartholinus discovered, are unequally refracted at the points where the ray enters and where it leaves the crystal. Any small object, such as a spot of ink on white paper, appears double when viewed through a calcite crystal. This property, termed double refraction, is possessed by all transparent crystalline substances except those which crystallise in the cubic or regular system. Of these so-called anisotropic crystals, some such as tourmaline of which C.S.H. writes, have the remarkable power of strongly absorbing one of the two beams while transmitting the other with but slight loss of light. Scientific discovery was not prosecuted with the same feverish haste in the 17th century as in modern times. It was twenty years before (he observation of Bartholinus was futher studied, this time by the Dutch scientist, Huyghens, who found that each of the two rays emerging from a calcite crystal could again be split into two in traversing a second but that this splitting varied with the relative position of the two crystals. He tried to explain this puzzling fact on the basis of the wave theory of light which he advocated, but failed, and finally gave it up. Newton, raised the question, “Whether the rays of light (in the two beams) are different in their sides?” in which remark of course, there is the gist of the true explanation, namely, a lateral polarity. DIFFICULT OF EXPLANATION. What made explanation so difficult to Huyghens, and, later, to Young in England, Fresnel and Arago in France, and other believers in the wave theory of light, was that they pictured the vibrations in a beam of light as taking place, like those of sound, along and not across the direction of travel. (Sound, of course, cannot be polarised). Fresnel it was who had the courage to break with tradition and proclaim his belief in the transverse nature of the vibrations, a mode of oscillation which clearly permits of a- difference between rays vibrating, say, in a vertical, and those vibrating in a horizontal plane. From that date, 115 years ago, the theory of polarised light has developed hand-in-hand with an ever-increasing variety of manifestations, all of which it has proved competent to explain. The field of practical application, too, has continually expanded, the most important implement being the Nicol prism, devised by William Nicol, of Edinburgh, in 1839. This is a calcite crystal so cut and recemented that only one of the two plane polarised beams can emerge. A single Nicol thus gives from any light source a beam of light practically 100 per cent, plane polarised; two such Nicols in line will completely transmit or completely block the passage of a ray according as they are set with their polarising planes, “parallel or crossed.” BIG POSSIBILITIES Let me add one or two items to the list of uses of polarised light which my colleague has mentioned. The strength of sugar solutions is

conveniently and accurately measured by passing a beam of polarised light through a tube filled with the solution, and measuring the amount by which the plane of vibration is rotated by the “optically active” molecules of sugar. Any other optically active substance, such as starch, many kinds of vegetable and mineral oil, &c., can be similarly estimated, and the polarimeter is widely employed in such various industries as the manufacture of soap, rubber, leather, jam, and condensed milk. Again, polarised light lends valuable aid to the maker of glassware, in that it can reveal the existance of otherwise imperceptible internal strains in imperfectly annealed ware; similarly, a model of a bridge, or other structure, built in clear celluloid, can be tested to discover places where the material is likely to be subject to excessive strain. The stereoscopic moving picture seems now at last a realisable proposition, and improvements in the technique of television are anticipated from the use of this new polariser. But of all prospective uses of Polaroid that of eliminating the deadly danger of blinding headlights is perhaps the most impressive.