International projectionist (Jan-Dec 1950)

FOR centuries man observed how light rays bend as they pass from air into water or from air into glass. Hundreds of men of an investigative turn of mind made observations and measurements in an effort to determine just what relationship exists between light rays before and after refraction. Although the bending of light is easy enough to observe, and although the actual measuring of the angles of the light rays affords no particular difficulty, the true explanation of what causes the bending and the method of precisely determining its extent were not found until comparatively modern times. From the days of Claudius Ptolemy, in the second century of the Christian era, until the 17th century, men observed the phenomena, or facts, of refraction, and from these facts tried to derive some rule or relationship that would enable them to tell in advance just what direction a given ray would take under given circumstances and which would co-ordinate all of the facts that had been observed. Mathematical Exposition of Law The true explanation was not made until about 1621, when Willebrord Snell of the University of Leyden, in Holland, discovered what is now known as the Law of Refraction. Stated mathematically, the Law of Refraction is this: The sines of the angles of incidence and of refraction are ina constant ratio, which is the same as the ratio of the refractive indices. Unimportant though it may appear, this statement is really one of the greatest scientific discoveries of all time. It is like a key that unlocks the door to a world of wonder and beauty never before dreamed of. To anyone who does not use the key it may seem like a small and insignificant thing in itself because it gives no hint of the knowledge and beauty it is capable of creating. Yet Snell's Law of Refraction has made optical science possible. It has reduced problems of lens and instrument design to mathematical certainty. It is directly responsible for the modern spectacle lens, with its untold blessings to mankind. It FIGURE 1 Refractive Index of Gl ass Willebrord Snell's Law of Refraction, discovered in 1621 — a law basic to the whole science of optics — makes possible the precise mathematical computation of modern lenses and optical instruments. By SCOTT STERLING Member, Scientific Bureau Bausch & Lomb Optical Company has made possible the modern microscope, mighty conqueror of diseases; the telescope, which has revealed new worlds to us; and the photographic camera, one of the most powerful means of conveying information, and of recording human progress, that has yet been devised by man. It is true that both the microscope and the telescope were invented before Snell's Law of Refraction had been formulated. Huygens, a great scientist of the 17th century, and the originator of the wavetheory of light, said: "To devise the telescope merely by thinking, and by the application of geometrical principles, without the help of a fortunate accident, a man would have to be super-human." This would indeed be true without a knowledge of the Law of Refraction, but with the accidental discovery of the telescope to begin with, and the Law of Refraction to explain its operation, the whole field of optical science was made possible. Refractive Index of Glass The refractive index of glass is defined as the ratio of the speed of light in air to its speed in glass. It is not necessary to measure the speed directly, as a reference to Fig. 1 will show. Let us assume that we have a beam of light from a distant source, such as the sun, obliquely incident upon a glass surface XY. AM and BN represent portions of wave-fronts, and ABC and MN are rays of light at right angles to these wave-fronts. In a case such as this we say that the light source is at infinity, that the wave-fronts are plane, and that the rays are parallel. The light rays AB and MN reach B and N at the same instant. At this instant the ray at N enters the glass and thereafter travels in glass, while the ray at B is in air, and must continue to travel in the straight line ABC until it reaches the glass surface. In the meantime the ray at N goes from N to 0, because the speed of light is less in glass than it is in air. The ratio of the speeds in air and in glass is the same as the ratio of the lengths of the lines BC and NO. Now, the speed of light through any medium is something which nature has predetermined and has firmly established for that substance; hence the ratio of the speeds of light in two unlike media is invariably the same — a constant quantity, n, to which the name "Refractive Index" or "Index of Refraction" has been given. When the refractive index is known, we can then tell what curves to put on the glass to produce a lens that will fulfill any given requirements. Means for Measuring Index The refractive index is determined by measuring the angles of incidence and of refraction with the spectrometer or the refractometer. The term "refractometer" in this connection has nothing to do with "refracting" the eye. It is used to mean "refractive index measurer." When the refractive index is to be measured by means of the spectrometer, a small wedge of the glass must be prepared with two polished surfaces, one of which is silvered. The measuring is done by what is known as "autocollimation," perhaps the most accurate method known. For measuring the refractive index with the refractometer, the glass sample is prepared with two polished surfaces at right angles to each other, neither one of which is silvered. The sample is placed on a dense glass prism, which is a part of the refractometer, and the refractive index is measured by "criticle angle," with the light at "grazing incidence." The measuring of refractive index is an important part of the work of producing a lens or an instrument. Every pot of glass made in the B&L factory is measured, and the job must be done accurately. This work is under the direct charge of the Scientific Bureau. New Kodak Film Divisions, Managers Managers of two new divisions of Eastman Kodak motion picture film department is announced by Donald E. Hyndman, manager of the department. Effective July 1, Gordon A. Chambers is named manager of the new Southern division and Kenneth M. Mason manager of the new Midwest division. Chambers will handle the Southern division from Rochester, N. Y., he said, while Mason will headquarter in Chicago beginning about the first of the year. Emery Huse continues as manager of the West Coast division and E. M. Stifle as manager of the East Coast division. INTERNATIONAL PROJECTIONIST July 1950