几乎所有的教材，不论是中文教材还是英文教材，不论是《大学物理》《电磁学》还是《电动力学》，都认为自由空间中平面电磁波的电场与磁场总是同相位的. 本文从麦克斯韦方程组出发，证明上述结论实际上并不总是正确的. 一般情况下，只能说自由空间中平面电磁波的电场与磁场是“同步”的，可以同相位，也可以反相位，取决于坐标系的选择.

It is stated in nearly all textbooks that the electric and magnetic fields of any plane electromagnetic wave in free space are always in phase,no matter whether the textbooks are in Chinese or in English,whether they are College Physics,Electromagnetics,or Electrodynamics. Based on Maxwell’s equations,we prove in this paper that the above statement is actually not always correct. In general case,we can only state that the electric and magnetic fields of a plane electromagnetic wave in free space are in step,which can be in phase or out of phase,depending on the choice of coordinate systems.