430 ENERGY MAY BE PROPAGATED BY VIBRATIONS Three-dimensional Pictures Are Made Possible by Polarizing Screens. Polarizing screens have made possible the projection of three-dimensional (stereoscopic) still and motion pictures both in black and white and full color. Two nearly, but not quite, identical pictures taken with Fig. 199. Control of light in a modern streamliner train by means of a Polaroid screen. (Courtesy of The Polaroid Corporation.) a "two-eyed" camera having the lenses separated by the distance between the human eyes, are projected one over the other, one in horizontally polarized light and the other in vertically polarized light. The viewer wears Polaroid spectacles in which the axes of the lenses are at right angles to each other, so that one eye sees only one picture, while the other eye sees only the other picture. Polarized Light Is Used in Peace and War. When the eyes of soldiers attempt to penetrate the glare of the sky in search of enemy dive bombers, when the sea is scanned for submarines, the sheeting that screens polarized light will prove useful. In gun sights, periscopes, range-finders, telescopes and in dozens of other ways, polarized light will be made to serve the military machine we are building. Polarized light, applied to eye examinations, has trapped persons who falsely claimed damaged eye sight in order to collect insurance. One young woman claimed that one of her eyes was damaged in an accident. The examiner gave her a pair of polarizing glasses with the planes of polarization of the two lenses at right angles to each other. She was asked to read from one of those scrambled alphabetical charts that eye doctors use. The trick in the examination was to first project the charts on a screen with vertically polarized light and then quickly switch from vertically to horizontally polarized. This means that the image can be seen by only one eye at a time. When the woman kept reciting as the illumination changed from vertically to horizontally polarized light, it proved conclusively that she was
POLARIZED LIGHT 431 seeing equally well through both lenses and, therefore, with both eyes. $50,000 claim for damages was thrown out of court. Crack railroad trains speeding over western plains are light-conditioned as well as air-conditioned. Light conditioning is made possible by windows of two polarizing discs with flat surfaces against each other. They are mounted in such a way that the outer disc is set stationary to block all reflected glare while the inner disc can be moved by turning a knob, whenever the passenger wants to reduce or increase the amount of light that enters the car. Exactly the same principle is used in new variable sun glasses which enable the wearer to reduce the brilliance of a scene simply by touching a button on the glasses.^ Her STUDY QUESTIONS 1. What is polarized light? 2. In what three diff'erent ways may light be polarized? 3. What is a Nicol prism, and for what is it used? 4. What is the nature of a Polaroid screen? 5. Explain how the glare from streets, snow, sand, or water is removed by Polaroid screens. 6. Explain how the glare from automobile headlights could be removed by the use of Polaroid screens. 7. Why are the axes of the visor and the headlights in the polarized-light system proposed for automobiles arranged parallel and 45° to the road? 8. Explain why the blueness of the sky depends upon the clarity of the air. 9. How can one prove that the light scattered by the molecules of the atmosphere is polarized? 10. Explain how the intensity of light may be controlled by the use of Polaroid screens. 11. Why do Polaroid day glasses intercept the glare from only horizontal surfaces? 12. What are the advantages in the use of Pola screens in photography? 13. Explain the use of polarized light in detecting strains in transparent materials. 14. Explain how three-dimensional pictures are made possible by Polaroid screens. 15. Show how polarized light was used to trap a woman who falsely claimed damaged eyesight. 1 From Science News Letter, November 9, 1940, p. 294.