THE "BURNING GLASS" 43 



Experiment 23. "The 

 Burning Glass." 



Apparatus: A lens at 

 least 3" in diameter. 



Materials : Bits of 

 paper, cloth, wood and 

 leather. 



a. Hold the lens flat 

 to the light as it comes 

 from the sun and catch the light, after it has passed 

 through the lens, upon a piece of paper. Move the lens 

 toward or away from the paper until the spot of light is as 

 small as possible. Tell what happens. Why is it best to 

 have the spot of light as small as possible? Repeat, using 

 cloth, wood, and leather in the place of paper. Which 

 do you think would become hot quicker, black or white 

 paper? Try it. Notice the dark ring around the bright 

 spot. Where has the light gone? 



Review Questions, 10. 



1. What is the difference between fading and 

 bleaching? 



2. What is a sunbath? How are they taken? 



3. What is diffused light? How can it be produced? 



4. Which is hotter, red light or white light? 



5. Give some examples of cold light. 



6. What part of the day is hottest? Why? 



7. What material becomes the hottest in the sun- 

 shine? 



8. How may we obtain a large amount of the sun's 

 heat in a very small space? 



