说明：  FDTD方法是把 Maxwell 方程式在时间和空间领域上进行差分化。利用蛙跳式(Leap frog algorithm)--空间领域内的电场和磁场进行交替计算，通过时间领域上更新来模仿电磁场的变化，达到数值计算的目的
(FDTD method Maxwell s equations in the field of time and space differencing. Leapfrog (of Leap frog algorithm)- the electric and magnetic fields within the field of space alternating the update time field to mimic the changes in the electromagnetic field to reach the numerical calculation purposes)