Phase Change by Reflection 3 



as the fundamental equations used for Hertzian waves should be 

 applied in developing the theory. 



METHODS 



The methods that may be employed for determining the phase 

 change produced by reflection from the surfaces of metals and 

 some other regularly reflecting substances are of two general 

 classes : 



1. Polarimetric Methods. — Methods of this class are not direct 

 but require an application of the theory to determine the phase- 

 change. The theory has not yet been sufficiently tested to accept 

 its results as conclusive, and consequently more direct methods are 

 desirable. 



2. Interferential Methods. — Of these, the method of O. Wiener^ 

 and that of G. Sagnac^ are the only direct ones. All other inter- 

 ferential methods are indirect, yet because of their greater con- 

 venience, they have been the more generally used. 



The Indirect Methods. — If a sufficiently thin wedge-shaped 

 piece of glass is illuminated with monochromatic light a series 

 of interference bands, due to the interference of waves reflected 

 from its front and back surfaces, may be observed over the plate. 

 If now the back of the plate be coated with some substance, for 

 example, silver, the eifeot is to shift the interference bands from 

 their original position toward the thinner or thicker portion of the 

 wedge, according to whether the phase change is a retardation or 

 an acceleration. 



By stripping off, then, a portion of the coating in a direction at 

 right angles to the bands, two sets of bands are obtained. Their 

 relative displacement serves as a measure of the relative phase 

 change. The phase change by reflection from transparent sub- 

 stances has been thoroughly studied, and theoretically the light 

 (electric vector) suffers no change of phase upon the reflection 

 from air into glass. Accepting this, measurements with the above 

 wedge should give absolute phase change in the transparent region 

 for glass. 



s O. Wiener, Wied. Ann., 40, p. 203, 1890. 



9 G. Sagnac, C. R., t. 154, p. 1346-1349, May, 1912. 



169 



