282 OUR PHYSICAL WORLD 



line if the former is to be seen. Light then travels out from its 

 source in all directions in straight lines. 



It follows from this law that the intensity of illumination 

 varies inversely as the square of the distance of the illuminated 

 object from the source of light. Cut a piece of card i inch 

 square and hold it 6 inches from a candle flame or small flash 

 light in a dark room. Its shadow on a large white card or 

 screen held at 12 inches from the light will be a square 2 inches 

 on each side, or 4 square inches in area. The light, therefore, 

 that covers i square inch at 6 inches from the source would 

 cover 4 square inches at twice this distance. If the screen 



FIG. 131. Diagram showing varying light intensities 



be held 18 inches away, the shadow will be 3 inches on each 

 side, or will cover 9 square inches. From the diagram (Fig. 131) 

 it is evident that this law follows mathematically from the 

 proposition that the area of the bases of similar pyramids vary 

 as the squares of their altitudes, which is easily demonstrated 

 by one familiar with geometry. 



Practical application of this law is commonly made in 

 measuring the relative intensity of illumination from different 

 sources of light. This is usually expressed in terms of candle 

 power. Thus we say that an electric light is a fifty-candle- 

 power light. The standard is a carefully made candle of pure 

 sperm, inch in diameter, that burns 120 grams an hour with 



