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B Double Slit Experiment: Detection around slit?

  1. May 18, 2017 #1


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    Dear All,

    I have a question on the double slit experiment. From the references I've read so far, they are focusing on the results of the detector AFTER the 2 slits.

    Let's say when photons are fired, how many % actually pass through the slits and get detected at the end? Would some (and how many) end up colliding at the barrier (marked in red) where the slits are, and would this result change if that barrier is a "detector"?

    If so, what pattern does it show there?

  2. jcsd
  3. May 18, 2017 #2


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    You can use mirror as the two slit barrier. Then you can collect reflected photons and detect them with separate detector. But there is no reason to expect any difference in double slit pattern behind slits. Interference can be observed when photon can reach particular detector by two (or more) different paths. If photons on different paths end up in different detectors there are no interference effects.
  4. May 18, 2017 #3


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    True, only a % of the photons pass through the slits. There is nothing about the barrier (as a detector or otherwise) that particularly make a difference as to whether the screen P shows interference or not. The pattern on the red barrier would be a blob around the 2 slits, intensity higher toward the center.
  5. May 18, 2017 #4
    Nice question. You could consider what happen if you replaced the slits with a barrier and vice-versa. (This would be similar to zonde's suggestion). There would be a pattern, but more difficult to see. The two patterns added together should produce the same picture as if there were no slits!
  6. May 19, 2017 #5


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    Dear Zonde, DrChinese and Jilang,

    Thank you for your detailed explanations. I have a further couple noob questions on the experiment I hope you can shed some light (or photons) on. =)

    Arrival Timing of Photons
    In a normal double-slit experiment like the above setup, do photons always arrive at the detector at a constant speed (basically, speed of light), or does the arrival time experience fluctuation (albeit an extremely minute one)?

    Since the interference pattern is a result of many troughs of (probability) waves, and troughs seem to have a certain order in its propagation (just like in waves, some troughs are in front and some behind), does that mean if a photon is detected at a position created by interference of troughs further back, it would actually arrive at the detector at a slightly later time?

    I understand that the "wave" is not physical, but if the interference resembles the characteristics of normal waves, wouldn't that also signify a correlation to the spatial and temporal sequence of troughs?

    Reflection Interference
    If we change some parts of P (please refer to photo in first post) to become a mirror, so that the photon is either reflected back or detected there, would the reflection's backward "wave" interfere with the forward "waves", or perhaps even cancel it out since it's an opposing "motion"?
    Last edited: May 19, 2017
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