Can any body tell me what is "The spectral content of a plane wave train"?
It could be anything, its unspecified. A plane wave describes a wave that propagates in one direction and is infinite in the other two. Such a wave could contain any frequencies.
It could mean a couple of things.
First, the reference could be to the temporal frequency content- if the wave is polychromatic or monochromatic. Any wave train that has a finite temporal duration will have a temporal frequency spread
Or, it could refer to spatial frequency spread- any wave train of finite spatial extent will have a spread in spatial frequencies.
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