To show this phenomenon in the ripple tank, we have taken 

 the boundary M to be a plane in three dimensions (reducing to a 

 line in two dimensions), the waves impinging on it at increasing 

 angles of incidence. The model used here (step model) is the 

 same as the one used in section 6 to illustrate Snell's law. 

 When the angle of incidence is 0* then the refracted waves are 

 parallel to the incident waves and the change in velocity mani- 

 fests itself as a change in wave length (in the refracting medium). 



When the angle of incidence is increased, reflection is ob- 

 served at the boundary and it is seen that the crests of the re- 

 fracted waves are fainter than the crests of the incident waves, 

 indicating that the energy which would have gone into the second 

 medium if there had been no discontinuity, is now contained in 

 the reflected waves (figure 14). The greater the angle of 

 incidence, the greater the reflection. 

 11. The phenomenon of total reflection ''^ 



Suppose that the second medium is not unyielding to the inci- 

 dent wave front and that total reflection is accomplished by merely 

 reaching and passing the critical angle. It is of interest to 

 investigate what happens to the "superficial" wave in the second 

 medium. 



Christian Huygens showed that the absence of a refracted wave 

 and the increased intensity of reflection for angles of incidence 

 exceeding the critical angle follow from his principle as simple 

 consequences. Therefore, from the standpoint of wave theory, 

 Huygens' principle is the natural starting point for the study of 



5. Raman, C.V. (1927): Huygens' principle and the phenomenon of 

 total reflection. Optical Society of London, Transactions , 

 vol. 28, pp. 149-l^oT 



34 



