Introduction
Wheeler's experiment consisted of a standard double-slit experiment, except that the detector screen could be removed at the last moment, thereby directing light into two more remote telescopes, each one focused on one of the slits. This allowed a "delayed choice" of the observer, i.e. a choice made after the presumed photon would have cleared the midstream barrier containing two parallel slits. The two telescopes, behind the (removed) screen could presumably "see" a flash of light from one of the slits, and would detect which path the photon traveled.
According to the results of the double slit experiment, if experimenters do something to learn which slit the photon goes through, they change the outcome of the experiment and the behavior of the photon. If the experimenters know which slit it goes through, the photon will behave as a particle. If they do not know which slit it goes through, the photon will behave as if it were a wave when it is given an opportunity to interfere with itself. The double-slit experiment is meant to observe phenomena that indicate whether light has a particle nature or a wave nature. The fundamental lesson of Wheeler's delayed choice experiment is that the result depends on whether the experiment is set up to detect waves or particles.
Read more about this topic: Wheeler's Delayed Choice Experiment
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