The Constants of a Lens. 



By 

 Aikitu Tanakadate. 



Assistant Professor of Physics, Imperial University. 



The curvatures of the faces and the index of refraction of the 

 substance of a lens, may be found by the following simple optical 

 method. 



I diagram, let P^ be the source of light and 1\ the focus 



due to one refraction, and p, p2 their respective distances from the 

 lens. Also let i\ and r.2 be the radii of curvature of the front and 

 back faces of the lens respectively. Then 



_L_-/^=_J£jzJ_ (1) 



Considering i\ as a new source of light we have, for the determination 

 of P3 the focus due to reflection at the internal back surface of the 

 lens, the equation 



1 1 _ 2 



^"*'K"~^ ^^' 



neglecting the thickness of the lens. 



This converging pencil of rays is refracted again at the front 

 face, and the real image is formed at P^ : its focal distance ^4 being 

 given by the equation 



J-_JL^_JL=_1 (3) 



