function [ lastThickness] = solveParaxialImageLocation(refIndex,thick,curv)  
% solves for location of paraxial images. the output is the location of the image surface from the last system surface. 
% trace a paraxial axial ray
y0 = 0;
u0 = 0.01;
[yf,uf] = yniTrace(y0,u0,1,length(curv)-1, refIndex,thick,curv); 
lastThickness = -yf/uf;


