Jacques_L mentioned mirror, then i remebered a special kind of mirror, Phase Conjugate Mirror. This is a part of Non Linear Optics. In Non Linear Optics the superposition principle does not hold.
http://en.wikipedia.org/wiki/Nonlinear_optics
Then i found out Schrodingers wave equation is linear and that's where superposition in Quantum Mechanics comes from.
Quantum superposition. In quantum mechanics, a principal task is to compute how a certain type of wave propagates and behaves. The wave is called a wavefunction, and the equation governing the behavior of the wave is called Schrödinger's wave equation. A primary approach to computing the behavior of a wavefunction is to write that wavefunction as a superposition (called "quantum superposition") of (possibly infinitely many) other wavefunctions of a certain type?stationary states whose behavior is particularly simple. Since Schrödinger's wave equation is linear, the behavior of the original wavefunction can be computed through the superposition principle this way.[4]See Quantum superposition
http://en.wikipedia.org/wiki/Superposition_principle
Here is an excerpt from another article on Quantum Superposition
The principle of superposition states that if the world can be in any configuration, any possible arrangement of particles or fields, and if the world could also be in another configuration, then the world can also be in a state which is a superposition of the two, where the amount of each configuration that is in the superposition is specified by a complex number.
Here is a portion of the second article.
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
What am i supposed to make of this? Can we be absolutely certain that subatomic particles behave in a linear fashion? If not, then how can we completely trust a linear theory to explain non-linear phenomena?
