Sometimes I find it hard to understand how energy is conserved in destructive interference. Consider two waves on a string which interfere destructively such that the string is at its rest position. Where is the energy contained? Edit: the string example is not very good at illustrating what I dont understand. Consider instead the following example. A plane electromagnetic wave approaches two planes. Now the wave is both reflected and travels through the plane to the next wave is reflected again. If we now choose the spacing of the planes such that the reflection on the 2nd plane differs by exactly a 180degree phase to the first reflected wave a situation is created where we have constant destructive interference in the resulting reflected wave. Where is the energy stored?