The motion picture projectionist (Nov 1931-Jan 1933)

12 Motion Picture Projectionist December, 1931 The How and Why of Lenses By Lloyd E. Harding In the following article, the author explains in non-technical language certain of the principles underlying the science of optics. The subject is treated in a novel and progressive manner calculated to show in a simple way the various functions of lenses. — The Editor. IT will be profitable for the projectionist, who naturally has a great deal to do with lenses, to know something about light rays and their actions upon striking and passing through various transparent materials. Usually he is told, or he reads, that a lens "focusses" the light at a certain point, or that the light rays are refracted to a certain de Fig. 1 gree by flint glass and to a different degree by crown glass, without understanding why light is affected by these different materials, so that a lens of one material produces a greater focal length than a lens of different material of the same size and shape. In explanation of this action of various transparent materials upon light rays, it may be said that it is due to the fact that the speed of the light ray is changed when it strikes any material, and that the harder or denser the material the more the ray is retarded and the lighter the material the more the speed of the light ray is increased. The Velocity of Light That light has a definite speed of travel is well known. Its measurement has been made by a number of scientists. One of the earliest was made by a man named Roemer in the year 1675. The results of all the various methods used check so closely that there is no possibility of error, the speed of light being determined as about 186,000 miles per second. Roemer's method of ascertaining the velocity or speed of light is interesting. Referring to Figure 1, you see a diagram representing the Sun, S, with the Earth revolving around it as designated by the circle, the Earth being shown at two different places in its path around the Sun, at A and at B. This path of the Earth around the Sun is called the Earth's orbit. To the left is seen the planet Jupiter, J., which also revolves about the Sun but at a greater distance from it. Around Jupiter travel four moons, only one of which is shown at M. The shaded cone seen at the left of Jupiter is the shadow thrown by Jupiter as it stops the light from the Sun. Now as the moon, M, revolves about Jupiter, J, it is visible from the Earth at all times except when it plunges into the shadow of Jupiter, when it can no longer be seen from the Earth. This is known as an eclipse of Jupiter's moon, M. These eclipses occur at fairly rapid and regular intervals so that the occurrence of each eclipse can be accurately predicted in advance. After many observations it was found that the eclipse of the moon, M, occurred 16% minutes earlier when viewed from the Earth in its nearest position at A than when viewed from its farthest position at B. The only conclusion it was possible to reach was that the 16% minutes difference represented the time it took light to travel across the Earth's orbit from A to B. As this distance was known to be 185,000,000 miles, it was divided by 990 (which is 16 y2 minutes times 60 seconds) and this gave the answer desired, approximately 186,000 miles per second. So much for the proof that light has a definite velocity. Refraction of Light You are told that when light strikes a lens it is refracted or bent. In the case of a convex lens, which is the type used in the lamphouse as a condenser, the rays are bent inward toward the center of the lens so that the rays gathered by the whole surface of the lens facing the arc are brought to a "spot" at the aperture. What happens to the rays to change their course so that they can be focussed to a single spot? Let us first take the case of light rays striking a body of water and see what happens to them. Water is a dense material and has a similar effect on light rays to that of glass. In other words, if the body of water shown in Figure 2-A were glass its effect on the light would be similar though more pronounced. A Practical Example In Figure 2-A let the light parallel lines represent a body of water, the surface of which is denoted by A B. Assume point C to represent the source of light and C D a ray of light. If the ray proceeding from C is perpendicular to the surface A B, upon striking A B at O it will continue on through the water from O to D in the same straight line without any change in its direction having taken place. C O is called the Normal because it is perpendicular to the surface A B. Assume now that we move the source of the ray from C to any other position, for example, to point E Figure 2-B, so that in striking the surface A B it forms an angle with the normal C O. The ray E O, upon arriving at O, the point where the two mediums join, will not continue through the denser medium in the same straight line to F, but will be refracted (bent) as shown by the line O H. Angles of Incidence and of Refraction The path of the ray will now be EOH; the part E O is called the incident ray and O H the refracted ray. The angle E O C is called the angle of incidence, while angle HOD is called the angle of refraction. When light passes from a rare medium to a dense medium the angle of refraction is smaller than the angle of incidence. Conversely, if the ray H O passing through water meets the surface of air at point O it will not continue in a straight line to J, but will be refracted (bent) away from the normal C D and take the path O E. Suppose glass was employed in place of the water in Figures 2-A and 2-B, then the refracted ray H O would be closer to D, and if the ray passed out of the glass into air it would be refracted further away from the normal than if it had passed out of the water. Propagation of Light Up to this point light has been spoken of as light rays. To explain c J/ /E =£g£^ir ?£^=ir" /7 D Fig. 2 why these rays are bent when passing from a rarer to a denser optical medium it will be necessary to digress and give a more complete explanation of the theory concerning the propagation of light upon the fundamental property of wave motion. A spherical wave is considered to remain spherical throughout its path of