Pourquoi une onde plane n'a pas de réalité physique
A plane wave such as ##\cos x## has no physical reality because it goes on to infinity in the y and z directions. It is, hoewever, a very good approximation in many situations, and easier to deal with than a representation of a more realistic wave.
The most important feature of plane waves is that you can represent any real wave packet in terms of Fourier decompositions of plane waves (Fourier series and transforms).
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