% this file plots rays backwards through a set of lenses
% this is used to determine the image space principle plane
% this file uses the functions 
%   free_space_b.m
%   lens.m

global xx yy a1

clf
hold 'on'

ho=1;   %the object height

%lens focal lengths
f1=50;
f2=16.67;

%spacings between elements
xo=50;
x1=25;
x2=20;

%the position/focal length array
xf=[xo,f1,x1,f2,x2];
%the type of each element in the array
type=['x', 'f', 'x', 'f', 'x'];

lens_h=2;   % this is the height of the line plotted for each lens

%ray 1
aa1=[ho;0];

%ray 2
aa2=[-ho;0];

%ray 1
a1=aa1;
xx=0;
yy=a1(1);

for lp=length(xf):-1:1
    if type(lp) == 'x'
        free_space_b(xf(lp))
    elseif type(lp) == 'f'
        lens(xf(lp))
        plot([xx(end), xx(end)], [-lens_h, lens_h],'k')
    end
end
plot(xx,yy)

plot([0, max(xx)], [0,0],'k:')


%ray 2
a1=aa2;
xx=0;
yy=a1(1);

for lp=length(xf):-1:1
    if type(lp) == 'x'
        free_space_b(xf(lp))
    elseif type(lp) == 'f'
        lens(xf(lp))
    end
end

plot(xx,yy)


hold 'off'