International projectionist (Jan-Dec 1935)

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* * * Light and Lenses * * * THE theory of light changes almost from month to month and it requires constant reading to keep up with the procession. It is safe to say that no ordinary reader can keep up with these developments. The laws governing lens action, however, are well established and have been for many years. The principles to be outlined here can be found in a book1 published in 1897 and which I studied in 1900—31 years ago. These dates should not surprise anyone, because the fundamental law governing the action of various shaped lenses were promulgated by Huygens about 1670, from which we can understand that it is not by any manner of means a recent development. I cite the antiquity of this principle only to emphasize how unnecessary has been the widespread misunderstanding of lens action in the minds of many of those working with them, particularly among members of our own profession and, unfortunately, in most of the books published for popular study. Figure 1 is a classic illustration to be found in almost all our books as illustrative of the course of light waves in projection optics. This drawing is the source of most of the misunderstanding current on lenses. It is what can be properly termed a shorthand drawing — telling the true story if one knows how to read it. In its original location in a textbook, following the explanation of the Huygens construction, it is clear enough; but when lifted into the popular books without proper explanation it does not tell a true story. Now let's get into the subject. We'll try not to be logical — which means that we shall try to make it interesting. It has been my experience that a logical presentation of a subject dried it out. What is light? I answer, "I don't know." Dr. Miller doesn't know; nor Dr. Michaelson. Even Einstein doesn't know. So, why should we worry about it? For purposes of discussing light in lenses it is customary to consider light as a wave-motion in the ether; but do not forget that there is no ether and that some scientists say that it is not wave-motion — so there we are chasing our tails again. To get started, let us say that a ray of light is a train of impulses projected through space by an excited electron. This train of impulses obeys certain laws — and it is a couple of these laws in which we are interested. ^'Theory of Physics" by Ames. From I. P., February and March, 1932 By VICTOR WELMAN. How big is a ray of light? We get into the habit of thinking of a ray of light as we do of a pencil line on paper, and we know, of course, that a ray of light is commonly expressed in this fashion. But it is not a ray but multiple rays — a million, perhaps. An electron is about a hundred-millionths of a millionth-of-an-inch in diameter. If one of these electrons is vibrating at such an amplitude that the resultant wave can be seen as light, hundreds of these waves could ride in a tube the size of a human hair and still leave ample elbow room. So, a line on a piece of paper represents not one ray but millions of rays of many frequencies which can be separated into their various frequencies, just as a filter can separate radio waves of various frequencies. Huygens suggested in 1670 that light waves emitted from a point source of light advanced in all directions, and that if a surface is drawn through all the points of vibrations in the same phase, this surface will be a wave-front and the direction this front is advancing is a ray (what's this? ... a ray of light is a direction only and hasn't any size No.5 at all. That's even smaller than I said it was). Now, Huygens says further that every point in that wave-front acts as a propagator of wave energy and starts a new wave-front, so that if we deal with wavefronts and not with rays, which are directions only, we can predict just what will happen when light passes through lenses. Now our drawing (Fig. 1), starts to take on meaning. It never did, in the original, intend to show the course of light rays but an envelope containing some wave-fronts. Figure 2 illustrates the principle. Take a wave-front at AB. Each point on that front propagates a new front, and if we draw a line through similar points on each of these smaller wave-fronts, we get a new wave-front A'B'. Now, when a wave-front goes through a hole we know from experience that it does not cut-off sharply around the edges but the small wave-fronts lap around the edges, although the main front passes on in its original direction. Wave Direction If these wave-fronts are getting larger, they are called diverging waves. If they are getting smaller, they are called converging waves. If they are progressing in a plane, they are called plane waves. Now, light travels in the air at one velocity, but slows down when traveling through glass. In light coming from the sun the wave-front has a radius of 90,000,000 miles. A small arc of a circle of that diameter would be, as far as we on earth are concerned, a straight line and those waves come to us as plane waves. Consider in this drawing (Fig. 4), a plane wave-front approaching a bi-convex lens. This wave-front hits the glass in the middle of the lens and that part of the wave slows down; as each part of the front hits the glass it slows down. Then the upper end of the wave-front gets out into the air and speeds up again. That part still in the glass holds back until it is all out, and the front is now converging and comes to a point which is called the principal focus of the lens. With a concave lens, the ends of the wave-front are slowed up first as they hit the glass, the belly speeds up first when it gets through the glass ahead of the ends, and a diverging wave emerges (Fig. 5). Suppose the one face of this lens were flat: it can be seen in the drawing that a diverging wave still would come out of the lens, but it would not be diverging so rapidly. By the way, where is the focal point of this lens? We described the focal point of the other lens as the point where plane waves were brought to a point, but here these plane waves do not come to a point. This is a negative lens, and the focus is on the opposite side of the lens from which the rays emerge and is called a virtual focus, as distinguished from the real focus of the other lens. I said that wave-fronts from the sun may be considered as plane waves. For all practical purposes, a light wave-front originating, say, 100 feet distant may be considered as a plane wave-front. The waves from your screen to the projection lens are diverging waves, and from the lens to the screen are converging waves; but for practical purposes they may be considered as plane waves. You prob [13]