In concluding this chapter, we will write them here. 



(1) The normal to any point on the reflecting surface, 



and the directions of both the incident wave front and the 

 reflected wave front at that point all lie in the 

 same common plane. 



(2) If the reflecting surface is of such a nature that 

 its every diameter is large compared with the wave 

 length, then the reflection is said to he regular, 

 i.e., the angle of reflection is equal to the angle 

 of incidence. 



(3) Surfaces whose diameters are smaller than the wave 

 length do not regularly reflect the sound waves, but 

 merely scatter or diffuse them. 



Chapter 3* Diffraction 



12. Beginning of modern diffraction theory 



In accordance with geometrical optics, rays of light are 

 straight lines wherever the index of refraction is constant. 

 With this observation in mind, it was thought that light which 

 emanates from a point source a great distance from an opaque 

 screen should give a sharply defined shadow, which is called the 

 geometrical shadow. Actually, it is observed that the light is 

 propagated up to the screen as if the screen were absent, but 

 once beyond the screen, light enters the geometrical shadow. 

 This phenomenon which violates the laws of geometrical optics 

 is known as diffraction. 



Christian Huygens was the first pioneer in diffraction and 

 those that followed him used his principle as a sprlngbroad to 



41 



