ON THE REFLEXION AND REFRACTION OF SOUND. 241 



Hence, we get 



2a sin e = 

 a 



2a cos e = - B, 



2/3 sin e t B, 



and, consequently, 



and 



tan e = 



This result is general for all fluids, but if we would apply it 

 to those only which are usually called elastic, we have, because 

 in this case 7 2 A = y* A, , 



But generally 



and therefore, by substitution, 



(11); 



because ^ , and - = tan 0. 

 7 a 



As e = e t , we see from equation (9), that 2e is the change 

 of phase which takes place in the reflected wave ; and this is 

 precisely the same value as that which belongs to light polarized 

 perpendicularly to the plane of incidence ; (Vide Airy's Tracts, 

 p. 362*.) We thus see, that not only the intensity of the reflected 

 wave, but the change of phase also, when reflexion takes place at 

 the surface of separation of two elastic media, is precisely the 

 same as for light thus polarized. 



Airy, ubi sup. p. 114, Art. 133, 



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