SIMPLE-HARMONIC WAVES. DIFFRACTION 247 



parallel, and equidistant slits in a thin screen, the fraction of 

 the total incident energy which is transmitted is found to be 

 1/(1 + A?/ 2 ), where k = STT/X, as usual, and 



, a + b , Trb 

 U __, ogsec ___ (7) 



where a denotes the breadth of an opening, and b that of each 

 intervening portion of the screen. As a numerical example, 



Fig. 75. 



suppose the wave-length to be ten times the interval a + 6 

 between the centres of successive apertures ; then even if the 

 apertures form only one-tenth part of the whole area of the 

 screen, 88 per cent, of the sound will get through. In the 



