eat的意思就是两个频率差
beat的频率f=f1-f2,这里f=3
这里400有可能是f1,也可能是f2
所以另一个频率=400+3 或者400-3 也就是403或397
再问： 我怎么在书上看到beat是由于两个波out of phase造成的呢？
再答： no, it comes from two waves with different freqencies
再问： so, what will happen if two electromagnetic waves are out of phase?
再答： if they have same frequency, we call it interference.and if they are out of phase, it's a destructive interference.
再问： can you explain the concept" out of phase" and" in phase"? the book I read is slightly misleading, anyway, I just cannot figure out them
再答： a wave can be express as sin(wt-kx+phi) "phi" is the phase we are talking about for two waves, sin (wt-kx+phi1) and sin (wt-kx+ph2) if phi1-phi2=2Pi, 4Pi, 6Pi, etc, we call them in phase if phi1-phi2=Pi, 3Pi, 5Pi, etc, we call them out of phase but no matter how, they should have same frequency w (or f)
