The science of light . freely through itas well. In the lower half of the diagram the screen Bhas been turned so that the slit is horizontal and willtherefore transmit only horizontal vibrations. Noneof the waves will therefore be transmitted beyond it all depends upon how B is turned as to whetherit is transparent or opaque to the waves. Waves likethose between A and B, in which the vibrations are all POLARISATION 73 in one direction (in this case vertical), are said to beplane polarised, the plane of polarisation being the planeto which the vibrations are all perpendicular. In thisca
The science of light . freely through itas well. In the lower half of the diagram the screen Bhas been turned so that the slit is horizontal and willtherefore transmit only horizontal vibrations. Noneof the waves will therefore be transmitted beyond it all depends upon how B is turned as to whetherit is transparent or opaque to the waves. Waves likethose between A and B, in which the vibrations are all POLARISATION 73 in one direction (in this case vertical), are said to beplane polarised, the plane of polarisation being the planeto which the vibrations are all perpendicular. In thiscase the plane of polarisation is the horizontal plane. The polarisation of light was first observed byBartholinus in 1670. He found that a ray of light whenpassed through a crystal of Iceland spar was split upinto two separate rays of equal brightness. Thus onlooking through the crystal everything appears passing the rays through a second crystal they areagain doubled, but now the rays are usually of unequal. Fig. 42. brightness, and the relative brightness varies as thesecond crystal is rotated. In two positions of the secondcrystal two of the rays disappear completely, so thatthere is in these positions no subdivision of the rays bythe second crystal. Perhaps the easiest way in whichto observe this is to place one crystal on some printedpaper and note that the print appears doubled. Holdingthe second crystal in the hand and looking through it, atthe first the printing appears quadrupled, but two of theimages are fainter than the other two. On twisting thecrystal round in the hand, while looking through it, onepair of images of the print grows brighter, and the otherfainter, until the latter disappears altogether. On con-tinuing the rotation they reappear again and grow 74 THE SCIENCE OF LIGHT brighter, the other pair gradually growing fainter anddisappearing in their turn when the crystal has beenturned through 90° from the first disappearance. Thisprocess is repea
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectlight, bookyear1910
