116 LIGHT 



the total area covered by the radiations at any given 

 distance from the luminous point is greater at a greater 

 distance from the point. And as the same rays have to 

 illuminate a greater area, it is clear that the intensity 

 of the illumination cannot be as great. In other words, 

 the farther a surface is removed from a luminous source, 

 the less brightly it is illuminated. 



Experiment 91. Place before a lamp a large screen of wood 

 or cardboard so that its surface will be 5 inches from the flame. 

 At a point nearest the flame cut a hole one inch square in the 

 screen. Now place another screen 5 inches beyond the first, so 

 that the rays streaming through the hole will all strike this 

 screen. Measure the illuminated spot and compare with the size 

 of the opening. The second screen is now twice as far from the 

 source of the rays as is the first. Move it to a point three times 

 as far (i.e. 15 inches from the flame), and again four times as 

 far. In each case measure the illuminated spot, compare with 

 the opening, and note the brightness of the illumination. Make a 

 general rule to apply. 



QUESTIONS 



1. What three different effects are produced by the radiations 

 from the sun? What do we call these radiations when they 

 affect the eye, causing sight ? 



2. Do any other bodies besides the sun give off light waves ? 

 What are such bodies called ? Do other bodies than the sun give 

 off heat radiations and actinic rays ? State any examples to prove 

 this. 



3. How are we able to see such bodies as do not give out light 

 waves of their own ? What are such bodies called ? 



4. What is a light ray ? What sort of a line do rays usually 

 take? Can a wave travel in a curved path? How then can the 

 sun's rays light a room into which they do not stream directly? 



5. What is meant by the ether? Is it known to exist? How 

 does it compare in density with any matter that we know ? 



