Editing Superposition principle (section)

===Departures from linearity===
In most realistic physical situations, the equation governing the wave is only approximately linear. In these situations, the superposition principle only approximately holds. As a rule, the accuracy of the approximation tends to improve as the amplitude of the wave gets smaller. For examples of phenomena that arise when the superposition principle does not exactly hold, see the articles [[nonlinear optics]] and [[nonlinear acoustics]].