where the author got a mathematic formula FDTD ?

if you can help me to have from where the author got a mathematic formula?

https://www.mathworks.com/matlabcent...g-3d-fdtd-code

formula: Material(4,3) = lineH/(Z0*lineW*dy);

i think try to use these relations may help you .....J (current density)= σE.....J=I/A.......I=line integral of (H.dl)......E/H=Z0.................

thanks for your reply
do you can suggest me the reference of those equations please ?