Double slit experiment w/ light shining thru one aperture only

[sqljunkey]

Hi,
Will there be interference if I point the laser thru one of the openings of the double slit aperture only while the other one is open?

And is the interference more pronounced or the same if I shine it thru both the apertures at the same time.

 

[PeterDonis]

point the laser thru one of the openings of the double slit aperture only

How would you do this?

shine it thru both the apertures at the same time

How would you do this?

 

[sqljunkey]

I found this video, from this very crazy guy, the gist of it is that he creates interference with 1 slit.

I guess my question is will there be interference if you shine the laser thru a single slit?

Watch at your own peril:

 

[jtbell]

Yes, in general we have interference even with a single slit. The light traveling through different parts of the slit interferes with each other. Usually we refer to the result as "diffraction."

With two (or more) slits, the light traveling through different parts of each individual slit interferes in the same way. Also, we have interference between the light coming from the different slits. The result is a pattern that has two components. One component depends on the widths of the individual slits, like the pattern that you get with a single slit; we often call this component the "diffraction" part of the pattern. The other component depends on the spacing (separation) of the slits; we often call this component the "interference" part of the pattern.

If the width of the slits is much smaller than their spacing (separation), the "diffraction" part becomes very broad so that it effectively "disappears" compared to the "interference" part.

You can see diagrams and photographs here:

http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/sindoub.html

http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/dslit.html#c1

Some more details for single slit diffraction: http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/sinslit.html#c1
And for double slit interference: http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/slits.html#c1

If you start with a laser shining through two closely-spaced slits, close enough that the diameter of the beam overlaps both slits, you generally get a pattern that combines the "interference" and "diffraction" components. Then if you carefully slide a razor blade or other sharp edge across the slits so as to block one of them, the pattern changes from the double-slit pattern to the single-slit pattern shown in the photo in the second link above. The gaps between the smaller spots "fill in", leaving only the broader diffraction pattern. I've done this many times as a classroom demonstration.

 

[ZapperZ]

The phrasing here needs to be clarified.

The pattern that we see with one slit is called the diffraction pattern, not interference.

With two slits, the interference pattern is modulated by the diffraction pattern. So this is a different observation than a single-slit diffraction pattern.

https://www.physicsforums.com/threads/a-view-of-the-double-slit-experiment.945168/#post-5981866

BTW, so far, this is all optics/classical wave, with no direct question on quantum physics yet.

Zz.

 

[Justin Hunt]

What would happen if you used entangled pairs of electrons and sent one of each pair through one double slit and the other through another double slit, but on one of the double slit experiments you measured which slit the electron was going through. The measured ones should not show an interference patter, but I am curious if the entangled one would also not show an interference pattern and if it matters in what order the events happen.

 

[RPinPA]

What would happen if you used entangled pairs of electrons and sent one of each pair through one double slit and the other through another double slit, but on one of the double slit experiments you measured which slit the electron was going through. The measured ones should not show an interference patter, but I am curious if the entangled one would also not show an interference pattern and if it matters in what order the events happen.

You can do electron interference experiments and you still get an "interference" pattern corresponding to wavelength of the de Broglie wave of the electron.


But I believe what is really happening here is not wave interference, but sampling of the electron's probability distribution. It is thus demonstrating the reality of the quantum mechanical wave function.

You can also do one-at-a-time photon interference experiments, and you still get the pattern. Again, I think what you're seeing is sampling of a probability distribution rather than classical wave interference.

(The single photon experiment begins about 2:45)

 

[ZapperZ]

What would happen if you used entangled pairs of electrons and sent one of each pair through one double slit and the other through another double slit, but on one of the double slit experiments you measured which slit the electron was going through. The measured ones should not show an interference patter, but I am curious if the entangled one would also not show an interference pattern and if it matters in what order the events happen.

Let’s stick with one issue at a time. I don’t know if the OP has understood the answers given to his original question.

Zz.

 

[Cthugha]

What would happen if you used entangled pairs of electrons and sent one of each pair through one double slit and the other through another double slit, but on one of the double slit experiments you measured which slit the electron was going through. The measured ones should not show an interference patter, but I am curious if the entangled one would also not show an interference pattern and if it matters in what order the events happen.

If the measurement means that it is also unambiguously clear which slit the entangled partner goes through (this is no necessarily so and depends on geometry), there will be no interference pattern in single-photon interference. However, entangled photons usually do not show interference in double slits anyway, because the light is spatially incoherent. The order of events never matters in this kind of experiment.

If you are interested in the details, look up "Spatial correlations in parametric down-conversion" by Walborn et.al. (Physics Reports 495, 87 (2010), free version here: https://arxiv.org/abs/1010.1236). Section 6.1. (around figure 23) discusses entangled light shining on two double slits in detail. If you are not aware that there is a difference between single photon and two-photon interference, you should, however, also read the fundamentals section.

 

[ZapperZ]

If the measurement means that it is also unambiguously clear which slit the entangled partner goes through (this is no necessarily so and depends on geometry), there will be no interference pattern in single-photon interference. However, entangled photons usually do not show interference in double slits anyway, because the light is spatially incoherent. The order of events never matters in this kind of experiment..

Interference measurement has been done with entangled photons,such as here:

https://www.nature.com/articles/srep04685

But again, this is now a different topic than what the OP asked.

Zz.

 

[Cthugha]

Interference measurement has been done with entangled photons,such as here:

https://www.nature.com/articles/srep04685

But again, this is now a different topic than what the OP asked.

Zz.

Well, sure you can get double slit interference if the quantity that is entangled is something else than the spatial/momentum degree of freedom. Photons entangled only in polarization or time bins or OAM may of course show interference. I assumed that momentum entanglement was implied in this thread as soon as Justin Hunt moved away from the OP's question and started talking about electrons, but it is of course way less ambiguous to mention that explicitly (and better to discuss this elsewhere and wait for the OP to tell us whether he understands the difference between diffraction and interference).

 

[sqljunkey]

ZapperZ I think I understood it. I couldn't figure out the purpose of the second slit very well. I guess it is to show that the interference doesn't happen at the aperture and instead happens somewhere else. it looks like by reducing the size of the slit the diffraction pattern becomes less visible and then opening another slit the interference pattern becomes very much visible.

 

[sqljunkey]

But then I guess, if you had two apertures open, shining a laser thru only one, if that can be done by aiming the laser just right, would that show interference as-if it was being shun thru both the apertures anyways? (is there an experiment like that?)

 

[ZapperZ]

ZapperZ I think I understood it. I couldn't figure out the purpose of the second slit very well. I guess it is to show that the interference doesn't happen at the aperture and instead happens somewhere else. it looks like by reducing the size of the slit the diffraction pattern becomes less visible and then opening another slit the interference pattern becomes very much visible.

But then I guess, if you had two apertures open, shining a laser thru only one, if that can be done by aiming the laser just right, would that show interference as-if it was being shun thru both the apertures anyways? (is there an experiment like that?)

I have no idea what you are getting at here. Did you read the link to my post above?

Light (or electrons or neutrons, etc.) going through just ONE slit will exhibit a diffraction pattern. Light (or electrons or neutrons, etc.) going through 2 or more slits will exhibit interference pattern. There is a difference!

I do not get this "shinning a laser thru only one" slit in a double slit scenario. Why do you think this will be different than shinning light through a single-slit case?

Again, you have not made your point on why you asked this in the Quantum Physics forum, because your question here is basic wave optics. I'm still waiting for the relevance here.

Zz.