LIGHT WAVES AND THEIR INTERFERENCE 



15 



the actual distance d or by d/\ wavelengths. We say, accordingly, that 

 the two waves differ in phase by d/\ wavelengths. We may also say 

 that the P wave has been retarded with respect to the *S wave by d/\ 



Fig. II. 2. The resultant wave {P + S) formed by the constructive and destructive 



interference of P and S waves of equal amplitude. Constructive and destructive 



interference occurs in A and B, respectively. 



wavelengths or that the S wave has been advanced with respect to the 

 P wave by d/\ wavelengths. 



If the ordinates y of the P and S waves are added, the wave (P + S) 

 so obtained produces the same physical effects as the combined waves 

 P and aS. The wave (P + S) is called the resultant wave. The physi- 

 cal process by means of which the P and S waves combine to form the 

 resultant wave (P + S) is called interference. In other words, two waves 

 interfere to form the resultant wave. 



The sum of two waves, i.e., the new wave that results from the inter- 



