Having used the classical electromagnetic field theory and combined it with the effective-field conception, in this paper we show that: 1) The real part θ of Faraday rotation φ in any kind of the magneto-optical (MO) material can be expressed in the fundamental form of θ=VLHi. 2) The differences in the MO properties of the different kinds of the MO media mainly result from both the characteristics and the size of their effective field Hi and especially, the different temperature properties in various media are discussed. 3) The relationship between the real part θ and the imaginary part ψ of φ can be described by the effective extinction coefficient η*. 4) The other MO properties are predicted. It is proved that the effective field Hv, which is concerned in both the spin-orbital interaction and the exchange interaction (or the indirect exchange interaction) in media, can be expressed in the form of Hv=vM (or E1=1viMi). Some theoretical results are verified by experiments.