[sciconoclast (OP)]

    Why does the double slit interference pattern persist in this experiment once the light path is known?

     If light first passes through a 0.20mm single slit and then, after traveling 2m, through a 0.20mm double slit with a 6.05 spacer, a pattern with double slit spacing and two separate center bans occurs.   After a distance of 2m more beyond the double slit a partial barrier can be placed blocking the left half of the pattern with the right edge of the barrier positioned on the centerline in the dark space between the two center bans.   The right center ban and peripheral bans will still display the single slit spacing on a screen 12m ahead.

     There is no shift to the pattern that would be formed if light was refracted around the barrier edge.   The center ban from the right slit actually passes at an angle behind the barrier and extends 5mm across the center line.   With the right slit blocked the single slit peripheral bans from the left slit are un-refracted and light is not detected within 15mm or more to the right of the center line.

     With all of the light in the remaining pattern after the barrier is in place clearly passing only through the right slit why does the pattern retain the double slit spacing?   What is going on here?

     Experiment Diagram

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[Carbonizer]

I think it's because it's still a double-slit experiment. You get the double-slit pattern only when the light passes through both slits, and that is what is happening here. This experiment is just forcing it to go through one before the other instead of both at the same time. (Does "at the same time" even have meaning to something traveling at the speed of light?)

[imatfaal]

Scico - you say 

 After a distance of 2m more beyond the double slit a partial barrier can be placed blocking the left half of the pattern with the right edge of the barrier positioned on the centerline in the dark space between the two center bans. 

Could you explain why you are getting a dark space in the centreline.  from my memory of this experiment at θ = 0 you should have full constructive interference.  when the angle is zero - your two path lengths must be equal and as your sources must be in phase then you should get light not dark.  I am thinking about the other results you have provided but I am concerned that I have got the wrong end of the stick entirely as I cannot even comprehend your set up :-)

Matthew

[sciconoclast (OP)]

Good Question imatfaal.

                          The single slit spreads the light so that when it passes through each double slit it is on divergent paths.   There is still overlap but there is two separate center bans each side of the centerline.  At the distance that the partial barrier is placed there is only a dark space between these two center bans

       It has been a general principle of quantum theory that there cannot be a separation of the field into dark and light bans until the quantum field interacts with the screen.   It is often thought of as a continues bow wave.   If you google the physicist Sharhriar Afshar you will find experiments demonstrating that the dark areas between the bans can be detected independent of the light bans at any point between the slits and the screen.  At first his experiments were considered to disprove quantum theory.  As usual in these instances a new principle " indirect measurement" was added to the quantum parameters.   By this principle the edge of the barrier test the quantum field for light in the dark area and the probability is not strong enough for photons to occur in this area [ which collapses this portion of the quantum field and removes these paths from any future contribution to interference patterns including diffraction around the barrier edge --  the comments in this bracket where added in a later edit for clarity ].     I am not saying that this is the correct explanation but what ever the reason the dark area is real.
                                         
                                                                     Thanks for the question.

Carbenizer: Thanks for your question as well.

        Actually the light does pass through both slits at the same time.   The anomaly is that only the light from one slit reaches a portion of the screen where there is double slit spacing.   In quantum theory there must be probable paths through both slits converging at the screen for a double slit pattern to be displayed.

                                                                  I hope this explains the nature of the experiment.

[sciconoclast (OP)]

Oh.
     Carbenizer, I just realized that you are saying the first single slit is part of the double slit.   This would be a violation of the Huygens-Fresnel Principle.