PHYSICAL FORCES 



99 



A 



(5) Reflexion. When considering radiant heat it is 

 convenient, since it always proceeds in straight lines, to 

 think of it as divided into an indefinite quantity of 

 straight lines or rays of heat as we before found it 

 convenient * to represent the action of gravity by a 

 number of parallel lines. 



As in dynamics we found that when a ball impinges 

 on a surface its angle of incidence is more or less 

 (according to its elasticity) equalled by its angle of 

 rebound, or of reflexion, so rays of heat will rebound 

 from a surface according to the same law namely, that 

 both the lines of impact and 

 rebound must lie within an 

 imaginary plane perpendi- 

 cular to the reflecting sur- 

 face at the point of contact. 



Thus if a ray A (Fig. 17) 

 falls at the point P, on a 

 reflecting surface, it will 

 be reflected to B, and a line 

 DP perpendicular to the sur- 

 face D will make the angle 

 DPB equal to theangleDPA 

 and the perpendicular, and both the incident and reflected 

 rays, will all lie in the plane CO. 



When two concave reflecting surfaces mirrors with 

 a certain definite curvature, are placed opposite each 

 other, a very curious effect may be produced. 



At a certain distance from each mirror is a spot 

 called its focus, all the rays radiating from which to 

 the surface of the adjacent mirror, will be reflected in 

 parallel lines. These rays impinging on the surface of 



* See ante, p. 43. 



