Abstract: Malus＇s law is verified by the experiment, and the Verdet constant of the sample is measured by using the points located in extinction and maximum optical power. According to the analysis of the result of the experiment, the question which existed universally in the experiment is discussed that the sensitivity should be tested when the optical power and it＇s relative change rate are the maximum. The conclusion is drawn that it is the most accurate to measure the rotating angle of the linear polarized light when extinction exists.

Key words: Malus＇s law, maximum change rate, maximum, sensitivity