IMAGES THROUGH SMALL OPENINGS 151 



translucent. Examples of translucent substances are 

 fog, ground glass, oiled paper, and thin china. Very thin 

 plates of all bodies, even metals, are translucent. Thus, 

 gold foil transmits a green light. 



167. Light and Its Properties. Light is the cause 

 outside of us that produces the sensation of sight. Sun- 

 light produces other effects besides the lighting of objects. 

 Thus, if the skin is exposed to sunlight, it is not only 

 illuminated, but also warmed. In addition, chemical 

 changes are brought about in the skin, and it is tanned. 

 The energy of the sun thus appears as heat, light, and 

 chemical energy. 



The velocity of light is very great: about 186,000 miles a second 

 (cf. 11). Sound waves travel through air at the rate of about 1,100 

 feet a second; but this is a snail's pace as compared with light. Be- 

 cause of the greater speed of light we see the flash of a gun before we 

 hear its report, and we see the lightning long before we hear the thunder 

 that accompanies it. 



Light travels in straight lines through a transparent substance, if the 

 density of the substance is the same throughout. A single line of light 

 is called a ray, and a number of parallel rays make up a beam. A 

 sunbeam is an example. If a sunbeam enters a dark, dusty room, its 

 straight path is shown by the illuminated dust particles. On a foggy 

 night a street lamp sends out straight lines of light in all directions. 

 These observations show that light travels in straight lines. The same 

 thing is shown by the formation of images and shadows. 



168. Images Through Small Openings. An easy way 

 to learn how images are formed is to let light pass through 

 the small opening of a " pin-hole camera. " The camera 

 (Fig. 141) consists of a box with both ends removed. A 

 hole is cut in one of the remaining four sides of the box, 



