596-600] Metallic Media 521 



598. We notice that if 1 + a =90, then Z'" = 0. Thus there is a 

 certain angle of incidence such that no light is reflected. Beyond this 

 angle Z'" is negative, so that the reflected light will shew an abrupt 

 change of phase of 180. This angle of incidence is known as the polarising 

 angle, because if a beam of non-polarised light is incident at this angle, 

 the reflected beam will consist entirely of light polarised in the plane of 

 incidence, and will accordingly be plane-polarised light. 



It has been found by Jamin that formula (566) is not quite accurate 

 at and near to the polarising angle. It appears from experiment that a 

 certain small amount of light is reflected at all angles, and that instead of 

 a sudden change of phase of 180 occurring at this angle there is a gradual 

 change, beginning at a certain distance on one side of the polarising angle 

 and not reaching 180 until a certain distance on the other side. Lord 

 Rayleigh has shewn that this discrepancy between theory and experiment 

 can often be attributed largely to the presence of thin films of grease and 

 other impurities on the reflecting surface. Drude has shewn that the 

 outstanding discrepancy can be accounted for by supposing the phenomena 

 of reflection and refraction to occur, not actually at the surface between 

 the two media, but throughout a small transition layer of which the thick- 

 ness must be supposed finite, although small compared with the wave-length 

 of the light. 



WAVES IN METALLIC MEDIA. 



599. In a metallic medium of specific resistance r, equations (A), namely 



KdX_dy 8/3 

 C~dt"~dy~dz ' 



etc., must be replaced (cf. 573) by 



G K d\ dy 8/3 



(568) ' 



etc. 



For a plane wave of light we can suppose the time to enter through 

 the complex imaginary e ipt and replace -- by ip. Thus on the left-hand 



of equation (567) we have ^- X, while on the left-hand of equation (568) 

 we have I - + T^-) X. It accordingly appears that the conducting power 

 of the medium can be allowed for by replacing K by K 4- . 



IjOT 



600. In a non-conducting medium the differential equation 



^^ = V X 



