424 ENERGY MAY BE PROPAGATED BY VIBRATIONS an iodine solution. This new screen transmits one-third more light than the previous Polaroid screen and polarizes 99.99 per cent of the light to which the eye is most sensitive. Light May Also Be Polarized by Reflection. When light strikes a glass plate at an angle, about 5 per cent of the light is reflected, and a portion of this reflected light will be polarized. A pile of glass plates not only produces polarized light by reflection Fig. 188. You can see perfectly through several thicknesses of Polaroid if their "optical slots" are parallel, as they are in the roll at the left. But when the "slots" of the material are crossed, as they are in the fold at the right, no light passes and you cannot see through. (Courtesy of The Polaroid Corporation.) but polarizes the transmitted light much as crystals do. The angle of a glass plate or mirror relative to the direction of the beam of light determines the relative amounts of ordinary reflected light and plane polarized light. The best angle, called the angle of polarization, is about 57 degrees for ordinary glass. Direct glaring reflections of sun and sky from the sea and asphalt or concrete pavements are polarized to a fairly high degree, while the diffuse reflections used in vision are not. By using a polarizing sun glass, it is possible to block the glare while transmitting the useful light. Light May Be Extinguished by Use Figure of Crossed Polarizers. 187 shows how a second Polaroid disk placed perpendicular to the axis of the first Polaroid disk will not permit the light which is polarized by the first disk to pass through the second disk. Figure 189 shows how the use of polarizing screens on headlights and windshields would eliminate automobile-headlight glare. Inasmuch as nearly 60 per cent of light is absorbed when it passes through a polarizing screen, headlights would have to be two or three times as powerful as present headlights when using polarizing screens. The benefits of polarizing screens would be proportional to the number of cars equipped with them. Each car is fitted with a windshield visor of Polaroid, with its "slots"
POLARIZED LIGHT 425 arranged at an angle of 45° to the ground in one proposed system of eliminating automobile-headlight glare. The headlights of each car are fitted with Polaroid lenses with their slots parallel with the visor slots, that is, at exactly the same angle. The light from the headlights strikes the road and is reflected back through the Polaroid visor to the driver, who gets the benefit of his own headlights because the visor and headlight slots are parallel. When two cars thus equipped meet each other, neither driver sees the direct light of the approaching headlights because the slanted slots of the Polaroid are crossed at right angles inasmuch as the cars are facing each other. Light from the approaching car cannot get through the visors, and glare is therefore eliminated. • Light May Be Polarized by Scattering. When light is reflected from particles such as the drops of moisture in the clouds, there is no polarization; but when light is Fig. 189. Polaroid visors used to eliminate headlight glare. (Courtesy of The Polaroid Corporation.) reflected from much smaller particles, such as dust and smoke particles, the light is scattered, and a bluish tinge typical of desert scenery appears. These small particles scatter short waves better than long ones; the blue waves are on the shorter-wave-length end of the visible Fig. 190. Unretouched photographs. Left, ordinary headlights. Right, Polaroid headlights through a Polaroid visor (Courtesy of The Polaroid Corporation.)