Based on the universal additivity law of the interference of light the following was achieved. (i) An analytic representation between the eigenvalue of the Kerr rotation angle &thgr;0kand the enhanced &thgr;kfor a double‐layered film, &thgr;k=en1&thgr;0k, was obtained theoretically, whereeis the enhancement factor andn1is the refractive index of the dielectric medium. (ii) An analytic representation between &thgr;kand &thgr;0k(&thgr;k=e013e123n2&thgr;0k) of the trilayered film [bilayered transparent media and monolayered magneto‐optical (MO) medium], was also obtained, wheree013ande123are the enhancement factors: The calculated &thgr;kvalue is in good agreement with experimental results fore=1. In general, the theoretical calculation shows that &thgr;kis related to the thickness of the transparent media. Thus, if one wants to get an optimum value for &thgr;k, the thickness of the first transparent film layer (such as a glass substrate) must be selected carefully. It also shows that if the variation of the thickness of the glass substrate is 5 nm, &thgr;kcould vary by 0.1° for MnBiCe films. (iii) The relation connecting &thgr;kwith &thgr;0k1and &thgr;0k2for bilayered MO films has been theoretically studied, where &thgr;0k1and &thgr;0k2are the eigenvalues of the Kerr rotation angle of MO materials 1 and 2, respectively. Finally, &thgr;kfor a Fe/Co film was calculated.