xi LIGHT 171 



original path is called the angle of deviation. With the centre I 

 and any convenient radius describe a circle, and from the points 

 where it cuts the incident and refracted rays draw perpendiculars 

 on to the normal as in the figure. Also drop a perpendicular 

 from the point R'. It is clear from geometry that RT' is equal 

 to the perpendicular let fall from the point where the incident 

 ray cuts the circle on to the normal. The ratio between the 

 lengths of R'P' and SP is constant for the same two media, e.g., 

 air and water, whatever the angle of incidence. This ratio is 



FIG. 82. To explain the Rules of Refraction. 



called the index of refraction. Its value for air and water is about 

 | ; for air and glass roughly f depending upon the kind of glass. 

 The laws of refraction are then : 



1. The incident ray, the normal, and the refracted ray are 

 all in the same plane. The incident and refracted rays are 

 on opposite sides of the normal. 



2. If a circle be described about the point of incidence, 

 and perpendiculars be dropped upon the normal from the 

 intersections of this circle with the incident and refracted 

 rays, the ratio of the lengths of these perpendiculars is 

 constant for any two given media. 



Effects of Refraction. The following experiments illustrate 

 further the phenomena and effects of refraction : 



EXPT. 167. Place a bright object, say a coin, on the bottom 

 of an empty basin, and arrange your eye so that the object is 



