REFRACTION OF LIGHT. 



397 



gradually increased the angle of refraction will be gradually 

 increased until it becomes 90, when the ray will graze the 

 surface of the water AM. If the source of light be still 

 further removed from (7, as to ?, the ray will be reflected 

 to r ( 591). For all media there is an incident angle of 

 this kind, called the critical or limiting angle, beyond 

 which total internal reflection will take the place of refrac- 

 tion. The reflection is called total because all of the 

 incident light is reflected, which is never the case in 

 ordinary reflection. Hence, a surface at which total re- 

 flection takes place constitutes the most perfect mirror 

 possible. The critical angle (with reference to air) is 

 48 35' for water; 40 49' for glass; 23 43' for diamond. 



(a.) From this it follows, as may be seen by referring to Fig. 295, 

 that to an eye placed under water, all visible objects above the 

 water would appear within an angle of 97 10', or twice the critical 

 angle for water. 



(6.) The phenomena of total reflection may be produced by placing 

 the bottle shown in Fig. 292 upon several books resting upon a table, 

 and inverting the card so that a beam of light reflected obliquely 

 upward from a mirror on the table may enter through the slit near 

 the bottom of the bottle, taking a direction through the water simi- 

 lar to the line IA of Fig. 296. When one looks into an aquarium in 

 a direction similar to rA, images of fish or turtles near the surface 

 of the water are often seen. 



(c.) Place a strip of printed paper in a test-tube ; hold it ob- 

 liquely in a tumbler of water and look downward at the printing 

 which will be plainly visible. Change the tube gradually to a 

 vertical position, and soon the part of the tube in the water takes a 

 silvered appearance and the printing becomes invisible. Show that, 

 in this case, the disappearance of the 

 reading is due to total reflection. By 

 dissolving a small bit of potassium bi- 

 chromate in the water, the tube will 

 have a golden instead of a silver-like 

 appearance. 



(d.) Fig. 297 represents a glass vessel 

 partly filled with water. Mirrors are FIG. 297. 



