108 



HOW WE LIGHT OUR HOMES 



EXPERIMENTS OR DEMONSTRATIONS THAT WILL 

 HELP ANSWER THE PROBLEM QUESTIONS 



Experiment 75. What is the relation of the intensity 

 of illumination to distance from the light source? 



Cut a hole one inch square in the center of a piece of 

 cardboard about six inches square. Darken the room. Hold 

 a flashlight one foot in front of the cardboard. Mark off in 



i iN.sa. 



A SO.. IN. 



9 sa IN. 



O.FEET 



3 FEET 



FIG. 164 



square inches another piece of cardboard about sixteen 

 inches square and hold it one foot back of the first card- 

 board. 



a. How many square inches of the second cardboard are 

 illuminated? 



b. Move the cardboard marked in square inches two feet 

 from the first cardboard. Now how many square inches 

 of the cardboard are illuminated? 



As in all other experiments, record notes and complete 

 the incomplete statements in your notebook. 1 



As the distance from the light source increases, the same 



amount of light is spread over areas. This means that 



each receives light. 



Experiment 76. How is your home lighted? 



Make a survey of lighting in your home. What methods 

 of artificial illumination are used at night? What is the 

 source of light during the day? Do you have indirect, 

 direct, or semi-direct lighting? What kinds of walls and 

 ceiling are in your home? If electricity is used to produce 

 light, what types of incandescent bulbs are in use? What is 

 the watt rating of the bulbs used in the living room? How 

 many bulbs are in use in the living room? What suggestions 

 can you make for the improvement of the lighting in your 

 home? 



OTHER INVESTIGATIONS WHICH YOU CAN MAKE 



1. Investigate the lighting system used in your school- 

 house ; your church ; an auditorium. 



2. Learn to read a gas meter. 



3. Learn to read an electric meter. 



READINGS WHICH WILL HELP ANSWER 

 THE PROBLEM QUESTIONS 



What is the relation of distance to the intensity of 

 light? The amount of light we receive from any source 

 depends upon the intensity (brightness) of the source 

 and the distance we are from it. From our everyday 

 experience we observe that the nearer an object is to 



1 See workbook, p. 39. 



a source of light, the more brightly it is illuminated. 

 If you have performed the first experiment described 

 in this topic, you will have learned that when an ob- 

 ject is moved twice as far from the source it receives 

 only one fourth as much light. If the object were 

 moved four times as far from the source of light it 

 would receive only one sixteenth as much light. We 

 see from this that the intensity of light de- 

 creases rapidly with increase in distance from 

 the source. 



How do the walls and the ceiling of a room 

 affect light? The amount of light reflected from 

 the walls and ceilings of a room depends upon 

 the color and the nature of the surface of the 

 decorations. The following table shows the per- 

 centage of diffused light which differently 

 colored walls and ceilings will reflect. 



Color 



Per Cent Color 



Per Cent 



Ivory white 76 



Ivory 72 



Lichen gray 70 



Pearl gray 70 



Primrose 67 



Satin green 67 



Chrome yellow 62 



Buff 59 



Shell pink 58 



Cream . . .56 



Pink 51 



Light orange 50 



Silver gray 50 



Bright sage green 43 



Tan 37 



Sky blue 31 



Cardinal red 27 



Brown 18 



Dark green 05 



Maroon 05 



A study of the table of colors shows that room 

 decorations vary greatly in their capacity to reflect 

 light. By selecting materials that reflect well we in- 

 crease the efficiency of our lighting system and re- 

 duce the cost. 



The nature of the reflected light must be consid- 

 ered, however, as well as the amount. Sufficient light 

 without glare is desired. Strong, glaring light puts 

 a strain on the eyes which may be prevented by the 

 use of soft, diffused light. To produce good indoor il- 

 lumination, decorations that will both reflect and dif- 

 fuse light well should be selected. 



Illuminating engineers have estimated that for pro- 

 per daytime illumination in any room the window 

 area should be about one-fourth the floor space of 

 the room. This is an important factor in proper light- 

 ing of both the home and the school. 



When new light sources began to replace the 

 candle, the light-giving power of these lighting de- 

 vices was compared to the intensity of light from a 

 standard candle. This candle is made from a solid wax 

 obtained from sperm whale oil and burns at the rate 

 of 120 grains an hour. The illuminating power of this 

 candle is called candle poiver. In recent years the candle 

 power used in this and several other countries has 

 been established by the use of standard electric-light 

 bulbs maintained at the Bureau of Standards in Wash- 



