We have developed a new approach (the LQR method) for calculating the reflectivity and transmission spectra of a multilayer optical material with N interfaces, as an alternative to the matrix method. The approach allows the inclusion of the effects of interface roughness by introducing a "rough" element between adjacent layers. For this purpose we have developed an empirical model, which describes the effect of interface roughness on an optical beam passing through or being reflected from an interface. An assessment of the interface roughness of a multilayer structure was carried out by fitting the experimental reflectivity spectrum of GaAs/AlGaAs multiple quantum well samples with and without oxidation of the barrier layers. The refractive index and the thickness of the oxidised layers were also obtained from the fit.