REFRACTION OF LIGHT. 



395 



The index of refraction for any two media may be found by divid- 

 ing the absolute index of one, as given above, by the absolute index 

 of the other. 



614. Laws of Refraction of Light. (1.) 

 When light passes perpendicularly from one me- 

 dium to another it is not refracted. 



(2.) When light passes obliquely from a rarer to 

 a denser medium it is refracted toward a line drawn, at 

 the point of incidence, perpendicular to the refracting 

 surface* or, more briefly, it is refracted toward the 

 perpendicular. 



(3.) W^^en light passes obliquely from a denser 

 to a rarer medium, it is refracted from the per- 

 pendicular. 



(4.) The incident and refracted rays are in the same 

 plane which is perpendicular to the refracting surface. 



(5.) The index of refraction is constant for the same two 

 media. 



615. Illustrations of Refraction. Put a small coin into 

 a tin cup and place the 

 cup so that its edge just 

 intercepts the view of 

 the coin. A ray of light 

 coming from the coin 

 toward the observer 

 must pass above his eye 

 and thus be lost to 

 sight. If, now, water be 

 gradually poured into 

 the cup, the coin will 

 become visible. The 

 rays are bent down as 

 they emerge from the 

 water and some of them 

 enter the eye. For the 



same reason, an oar or other stick half immersed in water seems 

 bent at the water's surface, while rivers and ponds whose bottoms 



FIG. 294. 



