3o6 



NINETEENTH CENTURY. 



PT. III. 



behind it. Those which meet in the middle of the shadow 

 have each travelled exactly the same distance with the same 

 p umber of waves, so they meet as in Fig. 48, and a strong 

 undulation is produced, causing a band of light But on 

 either side of the exact middle the rays will not have tra- 

 velled exactly the same distance, but one will have made 

 half a wave more than the other, so they will meet as in Fig. 

 49, and destroy each other, causing a band of darkness. 

 Outside these again the ray which has come the longer dis- 



FlG. 48. 



Fig. 49. 



Diagrams illustrating the Interference of Waves. 

 In Fig. 48 the waves, c d, meet in the same phase, and produce strong undulations. 

 In Fig. 49 the waves, c d, meet in the opposite phase, and interfere with each 

 other. 



tance has had time to make up another half-wave, so it meets 

 the other ray as a friend again, and both of them rising 

 together, a strong wave and a light band is the result In 

 this way they go on, first helping and then interfering with 

 each other, and thus making alternate bands of light and 

 darkness across the shadow. For this reason Young called 

 his discovery the * Interference of light: 



