level the wave crests are in a position nearly parallel to the 

 "shore." 



It must be kept in mind that the linear slope of the model 

 is not necessarily in the same ratio as the normal lapse rate. 

 However, this does not destroy the analogy. 

 9» Reflection by a plane rigid surface -^ 



Suppose an incident sound wave is made to impinge normally 

 on a plane surface which is absolutely unyielding with respect 

 to the slight variations of atmospheric pressure due to the inci- 

 dent wave. The wave, upon striking the "wall," will be unable to 

 proceed in the line of propagation, and the wall will react as 

 if it, and not the air, were vibrating with the given period and 

 amplitude, resulting in a reflected wave along the line of 

 direction. Hence, sound waves incident upon a plane rigid medium 

 and light waves incident on a totally reflecting surface, are 

 completely reflected, the incident and reflected waves having 

 the same phase at the rigid boundary. 



The more general case, that of a plane harmonic wave incident 

 obliquely on a rigid wall has been discussed mathematically 

 (Rayleigh, 194-5) » with the assumption that the incident and reflect- 

 ed waves have equal amplitudes and wave lengths. This being the 

 case, everything will be the same in planes perpendicular to the 

 lines of intersection of the two wave fronts, and therefore the 

 problem may be considered as a two-dimensional one. Let the angle 

 betv;een the incident and reflected rays be 2a (fig, 11), and let 

 the rays proceed with a velocity, a, in a direction perpendicular 



3. Rayleigh, Lord, (Strutt, J. W.), 1945: Th e Theory of So und. 

 New York, Dover Publication, Volume II, pp. l^-ll * 



26 



