Superposition Principle: When Does It Not Apply?

The superposition principle states that interference between two waves may be obtained by adding the wave functions. Under what conditions is this not true?

None of the above conditions would violate the superposition principle. The superposition principle holds true for all types of waves, as long as the waves are linear and the medium through which they propagate remains constant. In other words, the principle applies as long as the waves do not interact with the medium in a way that changes their behavior. Therefore, the superposition principle is always valid for coherent waves with the same frequency, regardless of whether there is constructive or destructive interference. Additionally, the linearity of the wave functions does not affect the superposition principle, as long as they are both linear.