246 LIGHT 



of reflection equals the angle of incidence). If an observer 

 stands at C, he will think that the point A of the candle is some- 

 where along the line CB extended. 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 mirror. 

 Why objects are visible. If the beam of light falls upon a 

 sheet of paper, or upon a photograph, instead of upon a smooth 

 polished surface, it is not reflected in one direction only, but 

 in many. The surface of the paper or photograph is rough, 

 and as a result it scatters the beam in every direction. It is 

 hard for us to realize that a smooth sheet of paper is by no 

 means as smooth as it looks. It is rough com- 

 pared with a polished mirror. The law of re- 

 flection always holds, however, no matter what 

 the reflecting surface is, the angle of reflec- 

 tion always equals the angle of incidence. 

 Beams of light reflected from a smooth body are all parallel ; 

 those reflected from a rough body are inclined to each other in 

 all sorts of ways (Fig. 116). 



Hot coals, red-hot stoves, gas flames, and candles shine by 

 their own light, and are self-luminous. Objects like chairs, 

 tables, carpets, have no light within themselves and are visible 

 only when they receive light from a luminous source and reflect 

 that light in such a way that it reaches the eye. We know 

 that these objects are not self-luminous because they are not 

 visible in a dark room. When light from any luminous object 

 falls upon books, desks, or dishes, it meets rough surfaces, and 





