[sciconoclast (OP)]

What is happening in this experiment?   Is the principle involved in this experiment the same as in the one posted earlier titled " Why does the double slit pattern disappear in this experiment? ". 

In this experiment light is first sent through a 0.20mm single slit and then after 1.25m passes through 0.50mm double slits separated by a 1.80mm spacer.   The two divergent paths from the double slit are refocused by mirrors, at 1.50m ahead, back over and above the bench to converge at a screen 5.30m from the mirrors.   This produces a double slit pattern at the screen.   

If a 1.47m barrier shield is placed directly behind the double slit between the two paths the double slit pattern is replaced by a single slit pattern.   The shield does obstruct some of the peripheral light paths but the center bans are unobstructed and they are the paths that converge at the screen.   So why doesn't the convergence of two probable paths generate a double slit interference pattern?
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[graham.d]

Good experiment. It has to be to do with the "peripheral" light paths. A photon's probability amplitude is spacially spread by the diffraction. If you intercept some of this you will (I think) change the probability amplitude of the remainder of the pattern. If it did not do so you would be able to detect which slit a photon was going through and still maintain an interference pattern.

All these experiments are nicely showing the QM effects of interference as opposed from simpler em wave interference without resorting to reducing light intensity to a single photon in the apparatus.

[JP]

All these experiments are nicely showing the QM effects of interference as opposed from simpler em wave interference without resorting to reducing light intensity to a single photon in the apparatus.

I might not quite be following, but how so?

[graham.d]

I think because the interference only occurs when the convergence of the two paths are such that the route taken by each path is indistinguishable at the point at which the interference is apparent - at the screen. Simple em wave interference would not care about the paths as long as the waves had the polarisation from each path and the distance travelled was shorter than the coherence length of the laser - the interference would be the effect of the phase only. I'm not sure about this, however; I'm just postulating that this may be the case.

[JP]

I think I see.  I was misunderstanding the experiment.  I would, however, be very worried if the prediction of the wave theory (diffraction) differed from what was observed.  This isn't a terribly complicated setup, and that would mean that the wave theory of light would fail in pretty basic situations--and we don't see that in reality.  I'm almost certain that if one were to work out the rigorous diffraction theory and compare it to the observations here, that they would match.  (It would be a bit of a pain to do, however.)

[graham.d]

This was my opinion to start with, JP. I thought that the behaviour of the experiments was just down to physical problems with the experimental set-up. If the results are taken at face value, though, then it is a bit odd. Intercepting part of the diffraction pattern from each slit should be the same as removing some of the photons. If they are absorbed then they won't be part of the system anymore and will not reach the screen; i.e. they won't produce the "no interference" pattern but simply reduce the overall brightness. Scicicoclast has taken trouble to ensure intercepted light does not reflect back into the system although there maybe some effect from diffraction around the end of the blocking screen. I am pushed to see how intercepting part of the diffraction pattern would affect the remainder, though, as you say, the calculations are not simple. I would have expected that the remainder of the pattern would still interfere with the remainder from the other slit or, from a particle perspective, that photons not absorbed en route would still produce interference. I don't think simple wave theory would change the frequency or phase of a diffracted wave if part is removed, as long as this part was not too large and dominant. 

[JP]

The angled mirrors make analyzing this rather difficult.  If the mirrors were removed and the detector screen were placed there, I would roughly expect to see interference fringes in the region that can "see" both pinholes.  Outside of that region, I would expect to see something that looks like a single-slit pattern from each pinhole.  If that shows something weird, then there's something I don't fully understand going on with the longitudinal shield... 

[sciconoclast (OP)]

Back with more info.

     I modified the experiment by moving the shield 60mm farther away form the double slit, slanted the shield 20 degrees off of vertical, and placed the target screen at the end of the shield.   A line from the inside edge of ether slit to the front end of the shied intersects the centerline of the experiment at 0-51'-34".   This shields an area of 2.20mm on each side of the shield from light paths from the opposite slit; confirmed by alternately blocking one slit.   

     This is enough to shield the center area of the patterns.   With the shield in place at this new location, which opens up the interference field directly in front of the double slits, the double slit spacing returns to the center area, and of course in the peripheral bans as well.   There is also an interference pattern perpendicular to the slanted shield sloping up and above the left center bans and down and under the right center bans which is the light diffracted from the shield.

                                     I hope this modification helps in the analysis.