140 



HOW WE USE LIGHT 



is diffused, because air is filled with countless millions of 

 tiny irregular dust particles. These particles divert the 

 rays of the sun out of their straight course and give diffused 

 light. Without the atmosphere to diffuse the sun's light, 

 the earth would look very different because contrasts 

 would be much greater than they are now. The whole 

 sky would appear black in the daytime except for the 

 luminous disk of the sun and the points of light made 

 by the stars. In winter our north windows would receive 

 no light at all. It is fortunate for us that the atmos- 

 phere with its particles of dust and moisture diffuses light. 

 Refraction of Light. A famous English philosopher 

 once noticed that if he put a coin in a cup and then stood 

 away from it so that he could just not see it, when he 



poured water 

 into the cup, 

 the coin came 

 into view. This 

 curious happen- 

 ing is brought 

 about by the 

 fact that light 

 travels more 

 slowly in a dense 

 than in a less 

 dense material. When light enters the water, it slows up 

 and is bent from its course. There is one exception : a ray 

 of light passing into another medium of different density 

 at right angles (90) to the surface between them is not 

 bent, but continues on in the same straight line. But 

 when any oblique ray of light passes from air into water, 

 it will be bent toward the perpendicular ; and when it goes 

 from water to air, it will be bent away from the perpendicu- 

 lar. The bending of light rays when they pass from one 

 transparent body to another of different density is called 



Which is the real and which the apparent position of the 

 coin? 



