dw.dw, Total energy: dwdW +dW,pdVAo'sin2 w(t-X) U the kinetic energy and potential energy of the infinitely small mass in the medium is the same. dw W= dy =PA0'sin2@(t-*) 7pfw'sm'og三d
k p dW dW Total energy： sin ( ) 2 2 2 u x dW dW dW dVA t k p the kinetic energy and potential energy of the infinitely small mass in the medium is the same. sin ( ) 2 2 2 u x A t dV dW w dt u x A t T w T sin ( ) 1 2 2 2 0
Average energy density 20 Energy flux:the energy pass certain cross section of medium in unit time
Average energy density： 2 2 2 1 w A Energy flux：the energy pass certain cross section of medium in unit time. u S u
Average energy flux: P=WuS Energy flux density(Intensity)(波的强度)： The average energy pass through a certain unit area along the direction that waves travel =wu= puA"@
Average energy flux： P wuS Energy flux density(Intensity)（波的强度）： The average energy pass through a certain unit area along the direction that waves travel 2 2 2 1 I wu uA
Interference of harmonic waves Interference(干涉) the various pattern caused by the superposition of harmonic waves. Coherence(相干 性)：the sources S are coherence when they have same frequency and fixed phase difference
Interference of harmonic waves Interference（干涉） : the various pattern caused by the superposition of harmonic waves. Coherence （相干 性）: the sources are coherence when they have same frequency and fixed phase difference S1 S2