PHOTOGRAPHY OF UNDERWATER EXPLOSIONS 221 



points S' or P' on the virtual ray. For clarity, the deviations indicated 

 are very much exaggerated over ones actually occurring. Coordinate 

 lines on screens placed as shown in Fig. 6.4(a) appear to be distorted out- 

 ward, as indicated in Fig. 6.4(b). 



The optical refraction effect just described has been used by Halver- 

 son (31) as the basis of a method for determination of shock wave pres- 

 sures in water. In this method a ruled screen of Incite in the position 

 of the points S, S' is photographed from point Q. The resultant image 

 of the displacement SS' on the photographic record can then be used, 

 together with the known radius of the shock front, distance of the 



SCREEN 

 P' 



IMAGE OF 

 SHOCK FRONT 



(a) (b) 



Fig. 6.4 Refraction by a spherical shock wave. 



camera, and separation of coordinate lines, to determine the angles of 

 emergence from the spherical front at point A. If the actual path AS 

 is approximated by a straight line an average index of refraction can 

 then be computed for the path by applying Snell's law at point A. 

 This average value, and average pressure deduced from it, has no im- 

 mediate correspondence with any point values, but the peak pressure 

 at the shock front can be obtained by extrapolation to the limiting ray, 

 the error in the straight line approximation vanishing in this limit if the 

 true pressure-distance relation is a smooth curve behind the front. 



The necessary geometrical relations for a spherical front have been 

 obtained by Halverson and need not be repeated here. Computations 

 made on the basis outlined for shock waves 14| inches from three 0.55 

 pound tetryl charges gave an average peak pressure of 17,500 Ib./in.^ 

 with a spread of =t500 Ib./in.^ This value is 15 per cent higher than 

 the figure from piezoelectric gauge similarity curves. Approximately 

 the same differences were obtained in another set of measurements with 

 a different explosive. 



The systematically higher values from refraction measurements 



