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We consider the Sommerfeld problem of diffraction by an opaque half-plane with a real wavenumber interpreting it as the limiting case, as time tends to infinity, of the corresponding time-dependent diffraction problem. We prove that the Sommerfeld formula for the solution is the limiting amplitude of the solution of this time-dependent problem which belongs to a certain functional class and is unique in it. For the proof of uniqueness of solution to the time-dependent problem we reduce it, after the Fourier-Laplace transform in $t$, to a stationary diffraction problem with a complex wavenumber. This permits us to use the proof of uniqueness in the Sobolev space $H^1$. Thus we avoid imposing the radiation and regularity conditions on the edge from the beginning and instead obtain it in a natural way.