The motion picture projectionist (Nov 1931-Jan 1933)

December, 1931 Motion Picture Projectionist 13 travel. This form of wave motion is conveniently shown as a number of equidistant concentric circles spreading out from a point of light in everwidening circles as shown in Figure 2-C. The single ray idea is derived from the theory that while the wave front has a certain curvature, each little particle of its surface is being continuously projected in straight lines along a radius of the sphere. The light wave then really exists and what we term "rays" are simply straight lines which show the direction in which the wave is traveling. The rays are always perpendicular to the front of the advancing wave and, in the instance of spherical waves, it is along a radius drawn from the point L, as previously stated. If we consider L to be a mere point the rays diverging therefrom would be termed a "pencil of rays"; on the other hand, if the point L is, for example, the sun, or any other distant luminous object far removed from the point of observation, the waves are for all intent and purpose without any appreciable curvature when they arrive at the point where they are to be a subject of study. In other words, they are considered to be parallel lines of light. Optical Density With this in mind let us consider Figure 2-D in which is again shown a body of water. Approaching this is a number of parallel lines called a beam of light. Since the water is of greater optical density than the air, the beam will have its velocity decreased upon entering the denser medium; conversely, if the light beam passes from a dense medium to a rarer medium its velocity will increase. Therefore, we will assume that the light beam E F G H is moving in the direction as indicated by the arrows, and as the light ray D E passes the surface 0 P its velocity will be less because it has entered a denser medium. Assuming the denser medium to be uniform, rays C F, B G and A H will all go through the same change in velocity as they pass the surface O P. When ray A H passes the surface the ray D E will have traveled through the water only from E to L, therefore the new wave front will now be L M N K, and since the direction of light is always perpendicular to the wave front the direction now taken by the beam will be that shown by the arrows in the water. Lenses and Light We are now ready to study lenses and the behavior of light when passed through them. Lens is the name given to a solid of glass, a transparent medium which, due to the curvature of its surfaces, is capable of diverging or converging rays of light that pass through it. So far we have only traced the light from one me H Fig. 3 dium into another; now we shall follow it from its source, to the medium, through and beyond it. For our first example a sheet of plate glass will be used through which we shall consider a ray of light to be passing. The ray from source A, Figure 3-A, on entering a sheet of plate glass at B, the faces of which are parallel, will be refracted toward the normal. After passing through the glass, and upon emerging into the air again at C, it will be refracted away from the normal by the same angular amount. The direction of the emerging ray C D will be parallel with the incident ray A B, as shown by the dotted line F C. This explains the reason why an object viewed through a sheet of plate glass appears somewhat displaced as to position but otherwise unchanged in form. Effects of a Prism Light passing through an ordinary window pane is refracted in a similar manner, but due to the thinness of the substance the change in position of the object viewed is too minute to be noticed. Now refer to Figure 3-B, a triangular glass prism, and again taking A as a source of light, the incident ray A B, on entering the prism at B, will be bent toward the normal and becomes the incident ray on the surface at C. On emerging into air it is refracted away from the normal in the direction C D. From this we find that a ray of light in passing through a prism as shown is always refracted toward the base of the prism. Further, if the line of vision coincides with the line C D, the candle will not be seen at A but will appear to the observer to be at F. Therefore, an object viewed through a triangular prism always appears deflected toward the top of the prism. Figure 3-C represents a rightangled prism which may be classed as a total reflection prism. The ray from source A entering the prism perpendicular to the side D E will pass through the prism without a change of direction until it strikes the face E F at an angle of 45 degrees, when it will be totally reflected and pass out of the prism normal to the surface D F. A prism of this shape, having polished surfaces, makes the best-known reflector. The Double Convex Lens Another type of lens we are interested in because of its use in motion picture projection is called the "double convex lens," shown in perspective Figure 3-D. As a boy, how many times have you used this type of glass to burn holes in leaves, paper, and so on, by focusing the sun's rays? If we place two prisms with their bases in contact and then fill the portion shown by the curved lines with glass, Figure 3-E, it is seen that a double convex lens is nothing more than a curved form of two prisms so placed. This can be brought out more clearly, perhaps, by a study of Figure 3-F, and noting the change in the light rays as they pass through what can be called an assembly of prisms with curved faces in contact. Keeping in mind what you learned concerning the behavior of light as it passed through the single prism as shown in Figure 3-B, we can see in Figure 3-F that parallel light rays originating, for example, from the sun, S, will, when they strike the upper half of the lens, be bent downward, while the lower half of the lens, acting as an equal number of inverted prisms, will refract the light upward. The light emerging from both the upper and lower sides of the normal converge at the principal focus F. Since we now understand the evolution of a double convex type lens we can study Figures 3-G, 3-H and 3-1, from which we shall learn the principal effects of lenses on light rays incident on their surfaces. (Continued on page 33)