12S POLARIZATION OF LIGHT. 



circumstance, from a sort of fanciful analogy which it presents with the relations 

 of the poles of the; magnet, has suggested the name polarization, to distinguish 

 this condition of light. 



An interesting experiment of Mains, illustrating the identity of the phenomena 

 of polarization by reflection, and polarization by double refraction, is the follow- 

 ing: Let a ray of liglit pass, at a perpendii-ular incidence, throitgh a crystal of 

 Iceland spar, undergoing division into two rays ; and afterAvards let these rays 

 fall at an incidence of 52° 4-5' on water, or of 54' 35' on glass. Let then the 

 crystal be turned in azimuth until the principal section coincides with th(! plane 

 of reflection. The extraordinary ray will cease to be reflected altogether, though 

 the ordinary ray undergoes reflection as usual. Turning the crystal once more 

 in azimuth, until the principal section is 90 ~ from the plane of reflection, the 

 ordinary ray will, in its turn, wholly cease to be reflected, and the extraordinary 

 ray will revive. 



Another interesting and very curious experiment by Brewster, analogous to the 

 foregoing, may be performed thus : Let the light of a candle or other luminous 

 object be polarized by reflection, and afterward received, at the polarizing angle, 

 upon a plate of plane glass, which has its plane of reflection in azimuth 90^ 

 fi'om the plane of polarization. It will, as we have just seen, be wholly trans- 

 mitted, so that, to an eye placed anywhere in the direction in which reflection 

 would ordinarily occur, the radiant will be invisible. The. eye remaining in 

 thi- position, let now another person breathe upon the glass plate, and instantly 

 the luminous object will appear, and will contuiue to be seen until the film cf 

 moisture left by the breath has evaporated. This is because the polarizing 

 angl for Avater is not the same as that for glass. 



The experiment may be varied and made still more striking by placing a 

 second plate by the side of the first, and adjusting this one to tin- polarizing 

 angle for water. The radiant will then be visible in the second plate, bur not 

 in the first. In this state of things, if both plates be breathed on simulta- 

 neously, the light in the second plate will be extinguished and that in the first 

 revived by the same breath 



It is only at the angles which have been mentioned that polarization by re- 

 flection is complete. But partial polarization takes place in reflection at any 

 anglf ; being zero at the incidences 0' and 90 ^ and increasing from those inci- 

 dences up to the polarizing angle. 



Light is polarized by reflection from all polished surfaces; but it is only in 

 the case of bodies whose indexes of refraction are in the neighborhood of 1.4 

 that the modification which it undergoes has the simplicity which belongs to the 

 examples we are considering. The index of water is 1.336, and that of crown 

 glass 1.48 to 1.53. 



It was the conclusion of Mains that the angle of polarization of a given body 

 is independent both of its refractive and of its dispersive power. Dr. Brewster, 

 however, demonstrated that this angle depends on the refi'active power; and is 

 connected with it by the law that '• the index of refraction of any body is the 

 tangcmt of the angle of polarization." 



From this law we derive one or two interesting consequences; first, at the 

 angle of polarization the reflected ray is perpmdicular to the refracted ray, for, 

 putting £ for the angle of incidence, p for the angle of refraction, and n fur the 

 index, the law of Snellius gives us %sin//=:sin: ; and the law of Brewster, just 

 mentioned, gives ?i in: tan;. Hence — 



siut 

 tan: sinp^z:^- — sin/) = sin:; or, sin^'--=cos;, and i-{-p^=^90^. 

 cos; 



Secondly, when light falls upon a transparent plate having parallel surfaces, 

 if the angle of incidence at the first sm-face is the polarizing angle, the angle of 

 incid'ence at the second surface will also be the polarizing angle for that surface 



