LIGHT 

 EXPERIMENT 41. RELATIVE VALUE OF DIFFERENT LIGHTS 



I. Set up a screen so that it will be perpendicular to a 

 line AB as shown in Figure 38. At the point O, a slight 

 distance from the screen, place a rod or some other opaque 

 object in an upright position. Put a lighted candle at some 

 point C, slightly at the left of O, but farther from the 

 screen. This light will cast a shadow of O upon the 



screen. Place four simi- 

 lar candles at some point 

 D to the right of O and 

 farther from the screen. 

 These will cast a second 

 image of O upon the 

 screen. If the two sources 

 j) of light are moved back 



FIG. 38. Comparing the illuminating power and forth, the two shadows 



will grow darker and 



lighter in shade. Move C and D until the two shadows 

 are side by side and of exactly the same shade. When 

 the shadows formed on the screen are of the same 

 degree of brightness, the illuminations which cause them 

 must be of equal strength. Measure carefully the distance 

 of the two shadows from the illuminations which cause 

 them, and compare these distances. 



When four candles are at D and one candle is at C, how 

 much farther from the screen must D be than C, if the 

 shadows formed are of equal brightness ? 



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