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[Double slit experiment] if we look back at slit, what we'll see?

  1. Apr 23, 2013 #1

    from picture, If we put camera directly at screen and look back at slit.

    move camera around fringe A, B, C .... what we gonna see?

    in bright fringe A, we should see light source through one of two slit,
    when in dark fringe, we should shadow between slit,
    and when in bright fringe B ... will we see another multiply light source?? through which slit?

    what about C D E F???

    will we see light source appear though slit again and again even we move camera only on direction?? (form A to F)

    you know what i try to say right? = ='
  2. jcsd
  3. Apr 23, 2013 #2

    Simon Bridge

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    That setup is unlikely to produce the interference pattern shown, you normally have to put light from a bulb through a single narrow slit first - but, ignoring that...

    At a bright fringe, the camera sees both slits illuminated.
    If the camera is set up carefully, it'll see an image of the bulb through both the slits.
    Draw the ray diagram to see.

    (note: this would be a classical approach - see post #3 below for the QM/particle approach.)
    Last edited: Apr 23, 2013
  4. Apr 23, 2013 #3


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    I think there might be some misunderstanding. You don’t see “the light through one slit”, that would be equal to measuring which slit, and then of course the interference pattern is gone.

    What you see is photons hitting the CCD in the camera, one by one, building up the picture.

    It’s easier to understand this if you look at the video below, where electrons are used, but the principle is exactly the same.

    Single electron double slit wave experiment
  5. Apr 23, 2013 #4
    than what we'll see?

    or just like #1 say? see light source through both slit?
  6. Apr 23, 2013 #5

    Simon Bridge

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    In the context of a camera as per post #1, the camera will record light from both slits. Remember how a camera works? In terms of classical ray optics, and wide-ish slits, the effect of the slits is to bend the light that passes through them. You can use the same ray-tracing as for other optical systems where light rays bend to work out where the images show up.

    In the context of counting photons ... you either get light or not and no information about which slit they each came from.

    This is a very well discussed phenomenon - have you tried using the search function?
  7. Apr 23, 2013 #6


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    Simon just gave you a good answer. Think of light acting like a (water) wave thru the slits.

  8. Apr 23, 2013 #7


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    Hey Devil, I seem to remember you had a link to an interactive double slit app that had a specific feature I would like to see. You could vary the angles on polarizers in front of each slit and see the effect on the interference pattern. Do you still have that baby?

  9. Apr 23, 2013 #8


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    Oh, I’m having trouble remembering what’s up & down in EPR right now... :smile: but the one thing I can remember is the Polarizer Applet, but it there’s no double slit...

    Polarizer Applet

    Or was it this de Broglie Bohm Quantum Double Slit experiment?


    But I’m afraid the homepage is down (at least for the moment).

    Or was it maybe something here?

    … gosh ... I need to clean/tidy my hdd ... get organized ...
    Last edited by a moderator: Sep 25, 2014
  10. Apr 23, 2013 #9

    sorry but another question.

    If we can see light source through both slit.

    when I slowly move fringe A to B C D.... E
    which direction light source will move to reappear and disappear?
    left? right? or popup from shadow between two slit?
  11. Apr 24, 2013 #10
    I see there is no any sort of detector placed so you won't see the 2 strips,due to the measurement,so light will stay in wave form until it hits the screen.You will see light from both slits with the same intensity and luminosity if you move from A to B a will be a bit more faded and B becomes more bright since the edge of the hole is covering some light,but since light comes in parallel with the emitter and perpendicular with the slit but when it goes through the hole it interferes with itself so the waves from the screen's perspective are not parallel anymore,so it would be less bright near the intersection point of the waves because they cancel eachother out and more brighter at the peak point of the wave.
  12. Apr 24, 2013 #11

    Simon Bridge

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    I'll go along with that.

    The two slits will always appear equally bright.
    As you move along the screen, the slits will get dimmer then brighter at the same time as you move out of one bright fringe, across a dark patch, then into the next bright fringe.

    Note: very narrow slits just look like a pair of light sources.
  13. Apr 24, 2013 #12


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    A point: I would remind the OP that there are two scales of answer to the question. If you are asking about what you would see on a photon by photon basis then for each photon observation you don't see an interference pattern, that only manifests in the aggregate of many observations. In the aggregate you get the same behavior as for a classical wave, i.e. whatever interference pattern should emerge.

    Here is how you calculate both the probability distribution of what you see photon by photon and in the aggregate. Keep in mind you perceive direction based on the location that the photon is absorbed on the retina (or camera's film, CCD, or CMOS chip). So you would take a point on the retina, "sum over all paths" back to the source through each slit. What you are summing is amplitude as it propagates with phase over the path. Some issues are the aperture (how dilated is your pupal?) relative to wavelength and slit dimensions.
    ...pause to consider...
    Firstly if you have a nearly pinhole camera, aperture comparable to slit size and wavelength of the light, you will get the equivalent of a single pinhole interference pattern on your retina at the amplitude for the value of the regular double slit interference pattern at the position of the aperture. That would mean you would see for many photons, the light coming from a wide range of angles concentrated near the front and with ring interference pattern. On a photon by photon basis you see random photons (speckles in the dark) distributed with probabilities following the mentioned pinhole interference pattern.

    In the case of a very large aperture relative to the wavelength (more typical) you would get ...[ pause to calculate...] well I balked at doing the full derivation, I have tests to grade. But I might work this out when I have time. My intuition says you get an image of the slits hence you would see photon by photon, a 50 50 chance of it appearing for each slit, as mentioned by other posts.

    In the intermediate case you should get an overlay of the two cases, your aperture diffraction smeared over the classical image of the two slits so you'd see the blurred image of two slits with some fringing determined by aperture diffraction. But again, photon by photon a point speckle randomly distributed over the mentioned patterns.

    [edit: Reminder to all, the quantum behavior will always be probabilistic point observations with distribution matching the classical wave diffraction behavior.]
    Last edited: Apr 24, 2013
  14. Apr 24, 2013 #13


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    The distance between the slits is small, so you won't really be able to resolve them from your camera. You'll see the screen, and a blurry light coming from the screen, but you won't be able tell which slit it's coming from.
  15. Apr 24, 2013 #14


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    You're making assumptions about dimensions beyond the obvious one that the slits are close relative to the wavelength. But the slits are multiple wavelengths apart and a camera with sufficient magnification and aperture should resolve them fine.
  16. Apr 25, 2013 #15

    Simon Bridge

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    Fair enough to separate answers into different regimes though. Sometimes the best answer is that the observation being inquired about could never be made. This time - though - there are ways to make the observation.

    ... and even so - it seems reasonable from the post to take the usual "physics idealizations" about what "see" means. The idealization is most obvious in relativity lessons where we ask what one observer "sees" on another's clock ... never mind that they may be many light-years apart by that stage.

    In that sense is sensible to talk about what one would "see" with ones back to the screen, looking towards the slits.
  17. Apr 30, 2013 #16
    sorry to push this up again, but I don't want to post a new topic.

    if photon can act like wave by passing both slit at same time to create interference pattern.

    is it only happen in small scale?

    if it a wave-like particle, it should create interference pattern at any scale regardless to size of slit.

    does anyone try put setup on large scale first (where it's not show interference pattern)
    than shrink size of slit and light source bit by bit (but not turn off) to see when or where normal pattern change to interference pattern?

    and, is it only happen in slit? what about two small hole?
  18. Apr 30, 2013 #17

    Simon Bridge

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    Photon does not pass through both slits at the same time so the question is meaningless as written - but I see where you are going:

    You do get interference patters no matter what the size of the slit though - yes.

    For a wide slit - you get a wide central maxima.
    The pattern appears around the edge of a shadow region. You can see it by holding a razor-blade to the light (google for images "diffraction around an edge")

    If there are two edges, you get an interference patters at both edges. If the edges are close together you get a slit (or a narrow barrier) and the diffraction patterns join up to the familiar picture.

    Also - as you intuit, for two small holes. Or just one small hole:
    Or even a big hole.

    The point of using slits is to make the pattern spread out in only one direction.
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