1. Introduction 



That light waves and sound waves have an analogy in water 

 waves can be seen from the two properties common to all waves: 

 (1) energy is propagated to distant points, (2) the dlstiirbance 

 travels through the medium in such a way that the fluid as a whole 

 is not given any permanent displacement. We shall see that whether 

 the transporting medium is air or water, these two properties are 

 common to the wave motion. 



Since all types of wave motion obey the equation of wave 

 motion, the solution of each problem differs only in the imposition 

 of the initial and boundary conditions. 



The ripple tank used In these experiments is essentially the 



same one used by Rix (194-9), although the dimensions are somewhat 



larger. 



"We have built a tank 3 feet by 2 feet in which we 

 produce ripples in tap water with a mechanically driven 

 vibrator consisting of a strip of plate glass. The wave 

 field is observed directly or photographed by means of 

 stroboscopic illumination, the wave crests acting as 

 lenses to focus the transmitted light in a plane at a 

 convenient height above the water surface. Refraction, 

 interference and diffraction effects are produced with 

 the help of obstacles made of glass or plastic." 



The ripple tank has proved its usefulness by facilitating 



the study of wave propagation. Its superiority over other methods 



of studying certain types of wave motion is seen by the fact that 



the actual progress of the water waves can be both directly 



observed and easily photographed. 



2. Experimental technique 



The ripple tank used in testing the models is an improvement 



