Abstract: By studying on the double frequency phenomenon in Faraday effect experiment, the incompleteness of a few textbooks＇ description on the location of this phenomenon is pointed out, and such incompleteness is the result of careless mathematical derivation. The polarization intersection angel θ0 is defined, and by mathematical analysis as well as experimental analysis, it is proved that the double frequency phenomenon can be produced when θ0 is equal to 0 or π/2.

Key words: Faraday effect, Malus law, polarization angel, double frequency, magneto-optical modulation