510 Mr. E. Edser on the Reflexion of Electromagnetic 



may then be taken to be either the tangential electric force 

 or the normal component o£ the vector whose velocity is the 

 magnetic force. 5 ' 



The conclusions arrived at by Sir Joseph Larmor have 

 been adopted by most writers on the subject of radiation. 

 Thus, in connexion with reflexion from a moving mirror, 

 Prof. Poynting * writes : — 



"The perfect reflexion [of light] requires that the resultant 

 disturbance at the reflecting surface shall always be zero. 

 Hence the amplitudes of the incident and reflected waves 

 must be equal at the surface/' 



Treating of radiation emitted by a hot body and reflected 

 from a moving, perfectly reflecting mirror, Prof. R. W. 

 Woodf writes : — 



" The amplitude, as we shall see presently, remains the 

 same, and the increase of energy density is due solely to the 

 fact that more waves are present in unit length of the train 

 after the compression than before/' 



In a recent paper % Prof. H. L. Callendar writes : — 



" It will be observed that the pressure and the work 

 result essentially from change of frequency caused by the 

 Doppler effect.'" 



3. The above quotations, to which many others might be 

 kidded, show that Sir Joseph Larmor 's reasoning has been 

 unanimously accepted with relation to the reflexion of the 

 electromagnetic waves which constitute light. Some sur- 

 prise may be felt at this general acceptance §, when it is 

 realized that the reasoning is utterly at variance with the 

 fundamental properties of electromagnetic waves. Thus, 

 let E be the instantaneous value of the electric field, and H 

 that of the magnetic field, at a point in the path of a train 

 of plane electromagnetic waves. Then if the waves are 

 travelling in free space, and E and II are measured in C.G.s. 

 electromagnetic nnits, the energy per unit volume in the 

 neighbourhood of the point is equal to 



Stt + 8tt ' 



where k denotes the dielectric constant of the sether, equal to 

 1/c 2 , where c is the velocity of light. Also, from elementary 



* 'Pressure of Light/ p. 86 (1910). 



t i Physical Optics/ p. 609 (3911). 



X " Thermodynamics of Radiation/' Phil. Mag. May 1914, p. 872. 



$ Sir Joseph Larmor's reasoning is applicable to the reflexion of sound 

 waves at a moving mirror, only if we ignore the motion impressed on the 

 medium by the motion of the mirror. 



