Forward and Backword representation of a plane wave

[abaset]

Hi all;
In a reflection and transmission problem of light off a dielectric slab or Fabry-Perot resonator, is it necessary to describe the wave in the slab as a sum of forward and backward wave or just a forward wave propagate in the relevant direction is enough. ?
knowing that in the above medium, the wave in the medium of incidence is Ei +Er with Er = rEi and the field medium of transmittance is Et = tEi

In this case, is there any difference between oblique and normal incidence.

any insight is very much appreciated