THE INTENSITY OF LIGHT 



349 



Any dark body whose surface reflects light appears itself 

 to be luminous as long as the source of light remains, but 

 grows dark again when the source is removed. This is 

 the case of the moon. At new moon, the moon is so situated 

 with respect to the sun that light is not reflected to the earth 

 and we cannot see it. At full moon, half of the moon's 

 entire surface reflects the sunlight, and it appears very 

 bright. 



If a candle is held in front of a mirror and we look into 

 the mirror, we see the candle behind it. We know that 

 the candle is not there but that its light is reflected by the 

 mirror in such a way as to make it appear to come from 

 behind the mirror. We see the candle by the light the mirror 

 reflects. 



If we wish to see whether the edge of a board is straight, 

 we sight along it. If we wish to hit an object with a bullet, 

 we bring the rifle barrel into our line of sight. We there- 

 fore feel, confident that if light is traveling through a uniform 

 medium, such as air usually is, it goes in a straight line. 



The Intensity of Light. Experiment 101. Take two square 

 pieces of paraffin about an inch thick, or better two squares of paro- 

 wax, and place back to back 

 with a piece of cardboard 

 or tinfoil between them. 

 When a light is placed on 

 either side of this apparatus 

 the wax toward the light 

 will be illuminated, but not 

 that on the other side of 

 the cardboard. (Figure 98.) 

 If lights are placed on each side, it is easy to see when both pieces 

 of wax are equally illuminated, or receive the same amount of 

 light. In this way the strengths of lights can be compared. 



Place a candle about 25 cm. in front of one side of this apparatus, 



FIGURE 98 



