170 



ELEMENTARY GENERAL SCIENCE 



circle elfg. Draw Im and gk perpendicular to ebf, and com- 

 pare the lengths Im, gk. Obtain the ratio for different 



QrC 



positions of the pins ; it will be found practically the same in 

 all cases in which the same material is used. 



Notice that the direction of the ray cd, emerging from the 

 glass, is parallel to that of ab. 



FIG. 81. Refraction of Light passing through Glass. 



These experiments will enable the general rules of refraction 

 to be well understood. 



Rules of Refraction. In Fig. 82 the shaded lower part of 

 the diagram represents a denser medium than the unshaded 

 upper portion. The word denser is used here, and in similar 

 connections, to mean optically denser, and must not be confused 

 with what has been said of the density of bodies in Chapter VII. 

 Let III represent a ray passing from the rarer to the denser 

 medium, or the ray incident on the surface of the denser medium 

 at I . The angle RI makes with the normal at I is the angle of 

 incidence. The ray is bent, as we have learnt ; instead of continu- 

 ing its course in a straight line along IR' it is refracted and 

 travels in the direction of IS which represents the refracted ray, 

 the angle SIP being the angle of refraction. The angle R'lS 

 which represents the amount the ray has been turned out of its 



