Fundamental formulas of magneto-optical effects are deduced according to classical theories concerning electro-magnetic fields. Obvious expressions of Faraday rotation θ that are applicable to various magneto-optic media are obtained by using the concepts of effective fields. It is proved that θ is proportional to the effective field H6 and has a strong dispersion nature. θ is obviously affected by the external stress in the weak magnetic media. The verdet constant V of diamagnetic media is independent of temperature T.