60 FIRST YEAR SCIENCE 



Experiment 31. Hold an ordinary spectacle lens such as is used by 

 an elderly person, or any convex lens, between the sun and a piece of 

 paper. Vary the distances of the lens from the paper. The heat and 

 light rays from the sun are bent so that they converge to a point. Try 

 the same experiment with a lens used by a short-sighted person, or a 

 concave lens. This lens does not have the same effect as the convex 

 lens. The rays are made to diverge. Why cannot long-sighted and 

 short-sighted persons use the same glasses? 



In the experiment of the penny in the dish, the water 

 in some way bent the ray of light and made the penny 

 come into the line of sight when it could not be seen before 

 the water was there. This experiment shows that when 

 light is passing from one medium to another it does not 

 always travel in the same straight line. Certain media 

 offer more resistance to the passage of light than others 

 and are called denser media. It is this resistance which 

 causes the bending of the ray. 



Suppose that a column of soldiers inarching in company 

 front are passing through a corn field and come obliquely 



upon a smooth open field. The 

 men as they come on to the 

 open field are unincumbered 

 by the cornstalks and will 

 move faster, and thus the line 

 of march will swing in toward 

 the edge of the corn field. It 

 Flg> 34 ' can easily be seen that the 



bending of the line would be in the opposite direction if 

 the soldiers were marching from the smooth field into 

 the corn field. If the company front was parallel to 

 the edge of the corn field, then the men would reach the 

 open field at the same time and there would be no swing- 

 ing of the line. 



The above illustration roughly explains what happens 

 when light passes from one medium to another. Refrac- 



