LIGHT 



223 



substances are later used as food by animals, and the 

 energy reappears as heat energy or muscular energy. As 

 we continue our scientific studies we will observe that 

 energy is never lost or destroyed. It is often changed 

 in form, but it never disappears. 



When a photographic plate is exposed to light, a chem- 

 ical change takes place. New substances are formed on 

 the plate in such a way that when properly developed a 

 picture may be produced on sensitized paper. It is this 

 same light energy which affects the various sensitized 

 papers used in photographic and blueprint work. 



Experiment 63. Intensity of Light. Figure 196 furnishes an 

 illustration of the way the intensity of light is affected by dis- 

 tance. Let L be a 

 small source of light 



and A a screen one L ,_--::rr^~" 



foot square, placed ^ v: -----U-.-"-._. 

 at a distance of four 

 feet from L. Since 

 light travels in 

 straight lines, the 

 area of the shadow 

 of A on B placed eight feet from L will be four square feet. 



From this experiment it is evident that the light which 

 is received by a surface having an area of one square foot 

 is scattered over an area of four square feet when the dis- 

 tance from the light source is doubled, and nine square feet 

 when the distance is trebled. A pupil who sits twice as 

 far from a lamp as another pupil, receives but J as much 

 light. 



How Light is Measured. --The amount of light given 

 by a lamp is commonly designated as a certain number of 

 candle power. A candle power is the amount of light 

 given off by a sperm candle known as " sixes," meaning 



FIG. 196. 



