WHY OBJECTS ARE VISIBLE 



III 



FlG. 62. The image is a duplicate of the 

 object, but appears to be behind the 

 mirror. 



105. Why the Image seems to be behind the Mirror. If a 

 candle is placed in front of a mirror, as in Figure 62, one of 

 the rays of light which 



leaves the candle will fall 



upon the mirror asA and 



will be reflected as BC (in 



such a way that the angle 



of reflection equals the 



angle of incidence). If an 



observer stands at C, he 



will think that the point A 



of the candle is somewhere 



along the line CB extended. 



Such a supposition would be justified from Section 102. But 



the candle sends out light in all directions ; one ray therefore 



will strike the mirror as AD and will be reflected as DE, and an 



observer at E will think that the point A of the candle is 



somewhere along the line ED. In order that both observers 



may be correct, that is, in order that the light may seem to be 



in both these directions, the image of the point A must seem 



to be at the intersection of the- two lines. In a similar manner 



it can be shown that every point of the image of the candle 



seems to be behind the candle. 



It can be shown by experiment that the distance of the 

 image behind the mirror is equal to the distance of the object 

 in front of the mirror. 



106. Why Objects are Visible. If the beam of light falls 

 upon a sheet of paper instead of upon a smooth polished 

 surface, no definite reflected ray will be seen, but a glare will 

 be produced by the scattering of the beam of light. The 

 surface of the paper is rough, and we can think of it as com- 

 posed of many tiny mirrors inclined in every possible direc- 

 tion. Each hump and hollow reflects its minute ray in its 



