XI 



LIGHT 



165 



The same facts can be demonstrated and shown to the whole 

 class in another way : 



EXPT. 163. Fasten a little whitened, wooden stick with wax 

 perpendicularly at the centre of a plane looking-glass. Cast 

 upon the mirror, at the foot of the rod, a beam of parallel rays 

 from the lantern, or a sunbeam coming through a hole in a 

 screen. Notice (a) that the reflected beam always makes the 

 same angles with the mirror and the stick as the incident 

 beam does, and (6) that the incident beam, the stick, and the 

 reflected beam all lie in one plane (Fig. 75). 



From these observations we learn that the light strikes the 

 mirror at a certain angle and leaves it at the same angle. The 



FIG. 75. The Laws of the Reflection of Light. 



angle at which the light or any sort of wave strikes the reflecting 

 surface is called the angle of incidence, and the wave an 

 incident wave. The angle at which the wave leaves this 

 surface is known as the angle of reflection, and the wave as it 

 leaves the reflected wave. 



Laws of Regular Reflection. From the experiments and 

 considerations which have just been described we arrive at the 

 two laws which must be very carefully remembered :- 



1. The line representing the reflected wave is in the same 

 plane with the normal and the line representing the inci- 

 dent wave, and is on the opposite side of the normal from 

 the incident line. 



