OPTICAL PROPERTIES OF CRYSTALS Hi:, 



appear, to a fish in the water, one and one half times taller than 

 lie really is; while the fish will appear smaller, as the rays follow 

 the same paths in the reverse direct inn. The angle aiOHi = r = 

 the angle of refraction. 



In passing from a rare medium to one which is more dense, the 

 ray is lient toward a perpendicular; and in passing from a den-e 

 medium to one which is less dense, the ray is bent from the perpen- 

 dicular. 



The angle of refraction will vary with the angle of incidence, 



but there is always a relation, as the value of - is a constant. 



smr 



In Fig. 310, in the two right-angled triangles ob"P and oTP, the 

 side oP, or hypothenuse, is common to both triangles. The angle 

 b"oP = noa = i, the angle of incidence, and TPo = Toni = r = 

 the angle of refraction. Pb" = v, the velocity in air, and oT = V], 

 the velocity in water ; then 



Pb" = oP X sin b"oP = oP X sin noa 

 = oP sin i, or v = oP X sin i ; 



oT = oP X sin TPo = oP X sin Ton 

 = oP X sin r, or v' = oP X sin r, 



v sin i 



or = . 



v sin r 



As the velocity of light of the same wave length is, in water, always 



Y 



the same, no matter what the direction, and likewise for air, -, 



, sin i 



and - are constant-. 

 sm r 



The ratio = n, the index of refraction of the water. When 

 sin r 



air is taken as the unit of comparison, and the velocity of light in 

 air is one, n, the index of refraction of water, is 1.333. 



An isotropic substance has a constant index of refraction, what- 

 ever the direction of the path of the transmitted ray may be, and 

 for water the index is 1.333. The indices of refraction of a few other 

 liquids and solids at room temperature and for yellow light are as 

 follows : 



Ether 1.35G 



Turpentine 1.472 



Hen/ene . 1.502 



