INTENSITY OF LIGHT 61 



tion is the name given to this bending of light in passing 

 through different media or through a medium of changing 

 density. Twilight, mirage, the flattening of the sun's 

 disk at the horizon and other appearances we shall find 

 later are due to this property of light. 



31. The Intensity of Light. Experiment 32. Take two square 

 pieces of paraffin about an inch thick, or better two squares of para- 

 wax, and place back to back with a piece of cardboard or tinfoil be- 

 tween them. When a light 

 is placed on either side of 

 this apparatus the wax 

 toward the light will be il- 

 luminated, but not that on 

 the other side o^ the card- 

 board. If lights are placed 

 on each side, it is easy to 

 see when both pieces of wax pj~ 35^ 



are equally illuminated, or 



receive the same amount of light. In this way the strengths of 

 lights can be compared. 



Place a candle about 25 cm. in front of one side of this apparatus, 

 and 4 candles, placed close together on a piece of cardboard so that 

 they can be readily moved, about 90 cm. away on the other side. 

 Move these candles back and forth till a position is found where both 

 pieces of wax are illuminated alike. Measure the distance of the four 

 candles from the wax. How many times as far away are they than 

 the one candle? 



The brightness of the sun's light is so great that even an 

 arc light placed in direct sunlight appears like a dark spot. 

 So great, however, is the sun's distance that the earth re- 

 ceives only a minute portion, less than one two-billionth of 

 the light and heat it gives out. It is impossible to express 

 the greatness of this light in ordinary terms. The stand- 

 ard measure for intensities of light is the candle power. 

 This is the light given out by a standard candle, which is 

 practically our ordinary No. 12 paraffin candle. The 



