magnetic field to an isotropic medium will induce the optical rotation property. In this

paper， light is regarded as an electromagnetic wave. By using the method of electrodynamics，

starting from the equation of motion of electrons in an isotropic me- dium under an external

magnetic field， the change of the system caused by the magnetic field will be analyzed. And then

the relative dielectric tensor is obtained， which is solved by combining with Maxwell

equations. By using the simple knowledge of electrodynamics， we can strictly deduce that the

eigenstates of magneto-optical system must be right-handed and left-handed polarization.

Furthermore， we can make readers better understand the source of fara-

day effect in the perspective of eigenvalue problem.