1312 EUGENE McDERMOTT 
spherical. At the contact plane between materials of definitely different 
physical constants, both refractions and reflections occur simultaneously. 
Part of the energy in the wave is transmitted through the second’ ma- 
terial in a direction different from its course in the first material, at the 
same time that some of the energy in the wave is thrown back as a re- 
flection. 
A very instructive illustration of these phenomena is given in Figure 
1. In the upper part of the figure a rope fastened to a wall at B is held 
at the other end in the hand. A quick movement of the hand at A starts 
a wave down the rope. This wave travels at a definite velocity for a 
8 
a FREFLECTION POINT A 
~ UNIFORM 
& 
a ~PEFLECTION POINTS Je A 
Bere 
N ¢ EAUE GHT 
Fic. 1.—Refraction and reflection of wave in elastic rope. 
specified weight of rope. On reaching B, most of the energy in the wave is 
reflected back toward A. A small part of the energy is refracted and 
actually causes the wall to move, though very slightly. This illustrates 
a very efficient reflecting contact between two media. 
If, as in the lower part of Figure 1, two ropes of different weights 
are fastened together at C, a somewhat more complicated example of 
the same phenomena presents itself. In view of this difference in weight, 
the velocity of a wave is different in the two ropes. A wave travelling 
down the light rope from its starting point at A is partly refracted and 
partly reflected at the junction of the two ropes, C, as well as at B. On 
reaching C a reflected wave starts back toward A and a refracted wave 
starts travelling toward B in the heavy rope at a slower speed than the 
original wave travelled in the light rope. On reaching B this wave is 
30 
