function abcd=slab(n1,n2,L)
% Returns the ABCD matrix for propagation through a slab (window)
%----------------------------------------------------------------
% Returns the ABCD matrix for propagation through a slab (window)
% with index of refraction n2. Index of external medium is n1. 
%
% SYNTAX: abcd=slab(n1,n2,L);
%
% n1 = index of refraction of external material
% n2 = index of refraction of slab material
% L  = Thickness of slab
%
%----------------------------------------------------------------
% SYNTAX: abcd=fdie(n1,n2,L);
%----------------------------------------------------------------

abcd=fdie(n2,n1)*free(L,n2)*fdie(n1,n2);