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Find the Fourier transform of a plane wave.
If students know about the Dirac delta function and its exponential representation, this is a great second example of the Fourier transform that students can work out in-class for themselves.
Students will need a short lecture giving the definition of the Fourier Transform \begin{equation} {\cal{F}}(f) =\tilde{f} (k)= \frac{1}{\sqrt{2\pi}} \int_{-\infty}^{\infty} e^{-ikx}\, f(x)\, dx \end{equation}
Students may ask what is meant by a plane wave. Help them figure out what is meant, from the context or give them the formula if time is tight.
Keep the time dependence in or leave it out depending on how much time you have to deal with a little extra algebraic confusion.