Has this double-slit experiment been performed? A weak, single photon, source is controlled by a fast shutter and directs photons toward a double-slit. Quite far beyond the slits, compared to shutter speed, is the photon detector (perhaps a film). Timed to the shutter action one of the slits is closed after the photon is calculated to be between the slits and detectors. 
Does the double-slit or single-slit pattern accumulate?
 A: Yes. This is called Delayed-choice Double-slit Experiment first thought by physicist John Wheeler.
Its result is that our present observations/ actions affect the past. When you observe/ close one slit after photon (or, any other Quantum denizen) has passed it (based on calculation), the two-slit interference pattern doesn’t form. It acts like it passed only from one slit.
A: Wave interference patterns are created with more than one opening for the particle to pass through. With only one slit, the wave passing through it would not meet another wave beyond the screen to interfere with it. Imagine it like the waves produced by dropping a pebble into a calm body of water. When the uniform ripples get to the screen with two slits in it the ripples (waves) are divided beyond the screen and are then two distinct waves that spread out and merge, causing the familiar interference as they blend together. The result is the particle riding a wave to an not assured place on the aforementioned film, with the center of the film being the most likely area that the particle will land. To get a good visual and a more detailed explanation visit Wikipedia's Double Slit Experiment.
