cir. xxxur. 



INTERFERENCE OF LIGHT. 



319 



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. 

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

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

 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 



FIG. 



FIG. 58. 

 Diagrams illustrating the Interference of Waves. 



In Fig. 57 the waves, c </, meet in the same phase, and produce strong undulations. 

 In Fig. 58 the waves, c d, meet in the opposite phase, and interfere with each 

 other. 



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

 each other, and thus making alternate bands across the 

 shadow. For this reason Young called his discovery the 

 ' Interference of light? 



If this experiment is made with light of one colour only, 

 as, for example, with light which has been passed through 

 red glass, and so is composed only of red rays, then the 

 bands are simply black and red. But if sunlight is used, 



