108 LIGHT 



102. How Light Travels. We never expect to see around 

 a corner, and if we wish to see through pinholes in three 

 separate pieces of cardboard, we place the cardboards so that 

 the three holes are in a straight line. When sunlight enters 



a dark room through a 

 small opening, the dust 

 particles dancing in 

 the sun show a straight 



FIG. 59. The candle cannot be seen unless the . , 



three pinholes are in a straight line. ray. If a hole IS made 



in a card, and the card 



is held in front of a light, the card casts a shadow, in the cen- 

 ter of which is a bright spot. The light, the hole, and the 

 bright spot are all in the same straight line. These simple 

 observations lead us to think that light travels in a straight 

 line. 



We can always tell the direction from which light comes, 

 either by the shadow cast or by the bright spot formed when 

 an opening occurs in the opaque object casting the shadow. 

 If the shadow of a tree falls towards the west, we know the 

 sun must be in the east; if a bright spot is on the floor, we 

 can easily locate the light whose rays stream through an 

 opening and form the bright spot. We know that light 

 travels in a straight line, and following the path of the beam 

 which comes to our eyes, we are sure to locate the light. 



103. Good and Bad Mirrors. As we walk along the street, 

 we frequently see ourselves reflected in the shop windows, in 

 polished metal signboards, in the metal trimmings of wagons 

 and automobiles ; but in mirrors we get the best image of 

 ourselves. We resent the image given by a piece of tin, be- 

 cause the reflection is distorted and does not picture us as we 

 really are ; a rough surface does not give a fair representa- 

 tion ; if we want a true image of ourselves, we must use a 

 smooth surface like a mirror as a reflector. If the water in a 



