I’m trying to work out how to apply the transmission line theory to optical systems. I understand that a transmission line model is modelled by characteristic impedance Z_0, Load Impedance Z_L and equivalent impedance Z_eq.

For example, if I have a normally incident ray on a dielectric slab with air either side of the dielectric slab. Which part equates to Z_eq, which part equates to Z_0 and which part equates to Z_L?

Thanks very much for your help