14 



AN ELEMENTARY THEORY OF PHASE MICROSCOPY 



theory of phase microscopy with Kohler ilhimination may consult the 

 Appendix, which is included as Chapter VII. 



2. LIGHT WAVES AND THEIR INTERFERENCE 



Light waves are regarded in classical physics as electromagnetic waves 

 whose electric and magnetic vibrations are at right angles to the direction 

 of propagation of the wave. Only the electric vibration is of direct 

 importance to the phenomenon of light. Consequently, a light wave is 

 usually represented as a transverse electric wave whose magnetic vibra- 



Fig. ill Representation of two light waves P and S of equal wavelength, X. 



The P and S waves vibrate in the same plane, YZ, and are propagated along Z. 



The amplitudes of the P and S waves are ap and as, respectively. The P wave is 



shown retarded with respect to the S wave by the amount rf/X wavelengths. 



tion is ignored. The height of the wave is chosen to represent the instan- 

 taneous value of the electric vector. 



Two related light waves are illustrated in Fig. ILL These waves are 

 traveling in the Z direction. Their electric vibrations are parallel to 

 each other and to the Y direction. The maximum height, Ug, of the S 

 wave is by definition the amplitude of the S wave. The P and S waves 

 are illustrated as having different amplitudes, ap and ttg, respectively. 

 The wavelengths, X, of the two waves are illustrated as alike and are 

 equal to the distance sqSi. The entire P wave lags behind the S wave by 



