Young's double slit experiment is often used to establish a level of credence in the Copenhagen interpretation of Quantum Mechanics, and the Heisenberg Uncertainty principle. The classic question is "Which slit did the photon go through to produce the interference pattern". The quantum argument says that when you try to determine which slit the photon went through in terms of a real number, you interact with the system and destroy the phase interference effect. The only "observable" is a real number probability distribution.
However, the wave field in the optics experiment contains both amplitude and
phase information, which can be manipulated to produce a distribution of the
slits from which the question which slit did the photon go through can be
determined.
The trick is to place a simple lens such that the double slit is at the focal
point of the lens. The phase shift and refraction produced by the lens will
recompose the phases of the wave field to produce the diffracted image of two
slits, not the classic interference pattern.
