LAW OF REFLECTION. 243 



departing is called the angle of reflection. It is the angle 

 formed by the line of this course and the same perpendic- 

 ular. 



The law, then, as stated mathematically, is, 

 That the angle of reflection is equal to the angle of in- 

 cidence, 



Which is only a more definite and precise way of say- 

 ing that the ray moves off from the point in the mirror 

 where it strikes in a direction just as far on one side as it 

 comes to it on the other. 



The diagram illustrates this very clearly. S S is the re- 

 flecting surface ; I o the course 

 of the incident ray, and R o the 

 course of the same ray after re- 

 flection. The line op being the 

 perpendicular, I op becomes the 



g % 3 angle of incidence, andpoH the 



DIAGRAM. LAW OF DEFLECTION, angle of reflection. The law is, 

 that in all cases, and whatever may be the direction in 

 which the ray I o comes, the line o R, into which it is 

 turned by reflection, will always be such that p o R shall 

 be equal to p o I. 



If this law is once understood and made familiar, you 

 will always see at once how rays will be reflected from 

 any surface the form and character of which you know. 

 If the ray comes to the surface in a line perpendicular to 

 it, it will be reflected back in the same line. If it comes 

 on either side of the perpendicular, it will be reflected back 

 with the same degree of obliquity on the other side. 



The action which takes place in accordance with this 

 law, in the case of a concave mirror, with light falling 

 upon it in one particular way, is shown very clearly in the 

 engraving on the following page. 



The point o being the centre of the curvature of the 



