2Q4 



OUR PHYSICAL WORLD 



off the arc de. With b as a center and a radius of 1.5, strike off 

 the arc fg. Continue the line ab toward c, and from the point 

 h where this line intersects the arc de erect a perpendicular to 

 the .surface of the glass and extend it until it intersects the 

 arc fg at i. Through b and i draw a line, and this will be the 

 course of the ray after refraction. It is evident that the ray ab 

 is refracted at b toward the perpendicular bj erected at b. 



Now suppose that the ray of light is coming out of a block 

 of glass with refractive index of 1.5 (Fig. 141). The ray ab 



strikes the surface of 

 the glass at b and 

 enters the air. If it 

 were not refracted, it 

 would continue toward 

 c. This ray, on enter- 

 ing the air from the 

 glass, will be refracted 

 away from the per- 

 pendicular. (Recall 

 the experiment with 

 the penny and bowl.) 

 To determine its 

 course, proceed thus : 

 With b as a center and 

 a radius of i, strike off 

 an arc de, and similarly a second arc/g, with radius of 1.5. 

 From h, the point where the extended ray intersects the arc fg, 

 drop a perpendicular to the extended face of the glass. This 

 cuts the arc de at i. Draw the line bi, and this will be the 

 course of the ray. One can readily judge whether the perpen- 

 dicular is to be dropped from the intersection of the extended 

 ray with the arc whose radius is i, or the arc whose radius is 

 1.5, by thinking whether the refraction is to be toward or away 

 from the perpendicular; and the experiment with bowl and 

 penny will recall this. If the refractive index of the glass were 



FIG. 141. Diagram to show method of finding 

 the path of a ray of light leaving glass. 



