The science of light . plane of the direction of the force pull-ing the electron depends simplyon the direction of its motion,not on the direction in whichits motion is handed on. The plane of polarisation is therefore turned in the samedirection, whichever way the light goes throughthe apparatus in Fig. 47, either right to left or leftto right. Lord Rayleigh shows how it is possibleto utilise this result so as to arrange an apparatuswhich will allow light to pass through in one direc-tion, while absolutely refusing to allow it in theopposite direction. The two Nicol prisms in Fig. 4
The science of light . plane of the direction of the force pull-ing the electron depends simplyon the direction of its motion,not on the direction in whichits motion is handed on. The plane of polarisation is therefore turned in the samedirection, whichever way the light goes throughthe apparatus in Fig. 47, either right to left or leftto right. Lord Rayleigh shows how it is possibleto utilise this result so as to arrange an apparatuswhich will allow light to pass through in one direc-tion, while absolutely refusing to allow it in theopposite direction. The two Nicol prisms in Fig. 47are placed so that the directions of vibrations whichthey will transmit are at 45° to each other, saythe directions OA and OB in Fig. 49. First pass thelight through the Nicol which transmits vibrations OAand then turn on the current in the electromagnet insuch a direction that the plane of polarisation is turnedin the direction of the arrow. Adjust the current untilthe light is completely extinguished by the second. 82 THE SCIENCE OF LIGHT Nicol. We know that the vibrations must now be inthe direction OC perpendicular to OB, they havebeen turned through 45° in the direction of the pass the light in the opposite direction. The lightwill enter the substance with its direction of vibration,OB. On passing through the substance its direction ofvibration will be turned through 45° in the direction ofthe arrow. The direction, on emerging, will thereforebe OA, and therefore the light will be completely trans-mitted by the second prism. We might thereforeconstruct a window through which we might see withoutbeing seen. The Kerr Effect.—Another magnetic effect on light isknown as the Kerr effect. Kerr discovered in 1887that when plane polarised light is reflected at thepolished pole of a strong electromagnet the reflectedlight is no longer plane polarised. It is found that thevibrations have become elliptical, instead of being instraight lines, owing to the reflectio
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectlight, bookyear1910
