%---------------------------------------------------------------
% Returns the ABCD matrix for free space propagation.
%
% SYNTAX: abcd=free(L);
%            <...> indicates optional argument
%
% L = distance to propagate.
% n = index of refraction. Default is 1.
%
% abcd = |1   L|
%        |0    1 |
%
%---------------------------------------------------------------
% SYNTAX: abcd=free(L);
%---------------------------------------------------------------

function abcd=free(L,varargin)

abcd=[
    1   L
    0   1
];