Are there any slit experiments where the photons only make two lines on the screen, as if they were little bullets fired though the slits? I have conducted many double and single slit experiments and they always show an interference pattern. I also know that when professional experiments are done in labs they get interference patterns too, even when only one photon at a time is sent though the slits; yet, many web sources talk about just two lines being formed in some test. Is that only for electrons and larger objects. It seems to me that photons never loose their interference pattern. Is this true?

  • $\begingroup$ Photons only diffract by interacting with the edges of the slits. If you make the slits substantially wider than one wavelength, then most of the photons will pass straight on through, like little bullets, to make two bright images of the slits. Any interference pattern from the few photons that are diffracted will go un-noticed because it will be so much less bright. $\endgroup$ – Solomon Slow Aug 19 '18 at 20:21
  • $\begingroup$ Right, maybe I should have acknowledged that in my question. Widening the slits is one way, but really that is just not doing a slit experiment. I mean at some point a slit just becomes an opening. So my question is meant to focus only on actual slit experiments where the slits a small enough to refract. $\endgroup$ – Lambda Aug 19 '18 at 20:27
  • $\begingroup$ @jameslarge see above message $\endgroup$ – Lambda Aug 19 '18 at 20:43
  • $\begingroup$ @jameslarge this statement is plain wrong: "Photons only diffract by interacting with the edges of the slits." $\endgroup$ – my2cts Aug 19 '18 at 22:06
  • $\begingroup$ I would also agree with James large. Interaction of the photons ( an EM wave) is primarily with the the electrons (EM field) near the edges of the slit or slits. $\endgroup$ – PhysicsDave Aug 20 '18 at 3:28

Yes slits always produce diffraction unless as Arpad points out the slit is modified so that the photon has a possibility of being absorbed. But then this is not really a slit. Another example, all camera lenses diffract based on aperture size.

  • $\begingroup$ Thanks for the answer. What then is all the discussion about the wave function collapsing and the photons or electrons acting like little bullets. Sometimes they are called quantum eraser experiments. $\endgroup$ – Lambda Aug 20 '18 at 4:42
  • $\begingroup$ Scientists tried to use a detector to figure out which slit the photon/electron went thru to help understand double slit diffraction. But any observation absorbs the particle and you are left looking at single slit diffraction. Wave function collapse is the same as saying the photon was absorbed. In some electron experiments, the electron can be absorbed but a lower energy electron can also be emitted which is a new wave function but has occurred after the slit so it's not a patterned (diffraction like) emission and has a typical straight line trajectory (bullet) like photons from a bulb. $\endgroup$ – PhysicsDave Aug 20 '18 at 13:58

First, photons can lose their interference pattern. If you put a filter on one of the slits, that creates inelastic scattering, and the photons will be out of phase, and there will be no pattern.

Please see here:


Now you are asking when there will be just two lines. If there is no diffraction or interference, there will only be two lines.

Please see here:


You need more than one photon to create the pattern. But one photon is shot at a time and to have interference, you only need one photon. One photon will travel as wave, and parts of the wave will go through the slits and the partial waves will interfere with each other. That will create constructive interference, that will be the bright parts in the pattern, and destructive interference, that will be the dark parts in the pattern.

It is the same with electrons or larger not elementary particles.

  • 1
    $\begingroup$ All photons (or electrons) that pass through the slit will make a bright spot, all will contribute to the pattern. The term interference implies cancellation which is an old historical explanation and is shown to be incorrect. The dark spots are caused by absence of photons, i.e. the photons due not travel to this part of the screen. $\endgroup$ – PhysicsDave Aug 20 '18 at 3:22
  • $\begingroup$ Interference occurs between overlapping portions of the photon wavefunction, not between photons per se. "Interference" simply refers to the addition of two complex wave forms, where the detected signal is proportional to the magnitude of the resulting sum. The rate of detection of photons at a point in space is proportional to that magnitude-of-the-sum. There is a simple experiment whose results contradict the statement that "photons due (sic) not travel to this part of the screen", unless photons can disappear in one place and reappear in another place without passing in between. $\endgroup$ – S. McGrew Aug 20 '18 at 4:23
  • $\begingroup$ Sorry, I misstated that: the detected signal is proportional to the square of the magnitude of the sum. The simple experiment is to set up two collimated, mutually coherent beams that cross in a region, completely overlapping. With a simple probe it's easy to prove that no photons are detected in planes that slice through that region. But downstream from that region, the two beams continue, having passed through those no-photon planes. You can say that no photons are present in those planes, but somehow they get from one side of each plane to the other side. Interference explains it. $\endgroup$ – S. McGrew Aug 20 '18 at 4:30
  • $\begingroup$ In diffraction experiments with slits all photons contribute to the bright bands. In many/some other light experiments, using the human eye which is not a square law director, you can perceive interference. Two photons out of phase will not be seen but their energy is still present, likely scattered and later absorbed. QM dictates the interaction of photons with the eye, and wave interference is a good way of explaining the observation but this is not what's happening in the double slit experiment. $\endgroup$ – PhysicsDave Aug 20 '18 at 13:42
  • $\begingroup$ Even thin films work with single photons (they still get reflected or absorbed based on wavelength compared to film thickness), 2 or more photons are not required. $\endgroup$ – PhysicsDave Aug 20 '18 at 13:43

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