DEVICES WHICH USE LIGHT 



111 



8. A device for measuring the intensity of illumination 

 of buildings is called a 



9. For proper illumination a schoolroom should have at 

 least foot candles. 



10. For proper daytime illumination the window area 

 of any room should be at least of the floor space. 



11. The unit of measurement of the power of an electric 

 current is the 



12. An ordinary electric lamp of the tungsten type re- 

 quires about watts per candle power. 



13. What are the most important factors to consider in 

 judging the illumination of any room? 



TOPIC 4. DEVICES WHICH USE LIGHT 



SUGGESTED PROBLEMS AND QUESTIONS 



1. What is a lens? 



2. What are the parts of the human eye and how 

 does the eye work? 



3. Of what importance is the proper care of the 

 eyes? 



4. How are the eye defects, nearsightedness, 

 farsightedness, and astigmatism, corrected? 



5. What are the parts of a camera and how does 

 a camera "take" pictures? 



6. Do moving pictures really move? 



7. What are some of the other optical instru- 

 ments important to man? 



SUGGESTIONS AND HELPS FOR STUDY 



1. Secure different types of lenses from your teach- 

 er and experiment with them. See Experiments 77 and 

 78. 



2. In connection with problem 5, if you have a 

 camera available, take it apart and examine the vari- 

 ous parts. Make a diagram showing how an image is 

 formed on the plate or film. See Experiment 79. 



3. Make a diagram of the human eye and compare 

 it with your diagram of a camera. 



4. Illustrate problem 4 with diagrams. 



EXPERIMENTS OR DEMONSTRATIONS WHICH WILL 

 HELP ANWER THE PROBLEM QUESTIONS 



Experiment 77. What kind of image is formed by a 

 convex lens? 



Hold a convex lens at a window. Move a piece of white 

 paper back and forth, back of the lens, until you find a 



place where an image of something out of doors is formed 

 on the paper. The distance from the lens to the paper is 

 the focal length of the lens. Measure this. 



Record the notes of this and other experiments in your 

 notebook. 1 



Experiment 78. How does a reading glass enlarge 

 print? 



Examine an ordinary reading glass. What kind of lens 

 does it contain? 



FIG. 169 



Examine the diagram in Figure 169. Explain how a read- 

 ing glass enlarges an object seen through it. 



Experiment 79. How is 

 a camera constructed 

 and operated? 



Examine carefully a fo- 

 cusing camera. Examine 

 the diagram (Fig. 170) and 

 note its essential parts. Ex- 

 plain how the image is 

 formed. Explain briefly the 

 functions of each of the fol- 

 lowing parts of the camera : 

 shutter, diaphragm, lens, 

 film or plate. 



FIG. 170 



Experiment 80. How can you make a simple tele- 

 scope? 



A simple telescope is easy to make if you have the follow- 

 ing materials : two lens holders, a meter stick, a thin convex 



1 See workbook, p. 40. 



