ECHOES OF BATS AND MEN 



very recent times have men observed the interactions of 

 radio waves with appropriate detecting instrmnents and 

 thus learned of their existence. Suppose that everything 

 in the world were suddenly made perfectly transparent 

 and, furthermore, that nothing gave off light or caused 

 it to change direction in passing from one material to 

 another. In such a world one might as well be blind. 

 Even though you possessed the only sense organ or de- 

 tecting instrument capable of receiving a type of radia- 

 tion that penetrated everything else in the universe, such 

 special powers would be of Uttle use. Because light and 

 sound do interact with matter around us, they are types 

 of radiation for which sensitive detectors are useful. This 

 is why animals and men have come to possess such 

 effective eyes and ears. 



The Nature of Sound Waves 



Wave motion can conveniently be thought about in 

 pure and continuous form: a sound having a single fre- 

 quency, for instance 2000 sound waves or cycles per 

 second, or light of a pure spectral color, for instance the 

 D line of the sodium arc having a frequency of 5.1 X 

 10^3 c.p.s. (51,000,000,000,000 c.p.s.). Such continu- 

 ous waves may be described quite accurately for most 

 purposes by a graph in which the size or amplitude of 

 the wave motion is plotted on one axis and time on the 

 other axis. For sound of a single frequency or a single 

 spectral color such graphs are smoothly undulating Unes 

 called sine waves. A sine wave is the graph you would 

 draw if you plotted the vertical motion of the hand of 

 a clock as a function of time. Suppose you tied a string 

 to the end of the hour hand of a wall clock, such as the 

 one in almost any schoolroom, and tied a light weight 

 to the other end of the string (Fig. 2). At nine o'clock 



36 



