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Young's double slit - help please!

Discussion in 'Science' started by physics_suits_you, Jul 5, 2011.

  1. If there is ONE demonstration in the entire physics spectrum which calls for good equipment, it is this one.
    I remember my teacher telling me (1965) that I should be able to see Newtons Rings when using sodium lamps, or diffraction patterns when using a spectrometer, but it was only the regular "smacks" which persuaded me to agree with him.
    This is THE classical indicator that light is a wave (as required for Y11 - I do know about wave-particle duality, etc). I have seen so many teachers TELLING pupils (Y8?) that "light is a wave" but not being able to offer any evidence - that is not science IMO.
    Get onto your head-teacher and say you MUST have a laser but consider Risk Assessments and please don't use it to show "light travels in straight lines".
  2. PinkHelen

    PinkHelen New commenter

    Cheers for the replies - I agree that this is one demonstration that needs to be done well. I still remember being shown this in school myself and seeing the completely unexpected pattern appear on the screen was wondrous to me - I want that to happen in my class next year too, which is why I've insisted to my department that we get the equipment for it. And I think we've finally got the correct equipment :)
  3. Agree with comments above about necessity of correct equipment. We have laser pens - a red & a green - plus a much older red laser that has to be wheeled in on a trolley (well almost).
    We use if for A level & there is always a 'wow' moment when the red & green fringes fall in different places. A dark room is essential. There are some pretty amazing pictures on the web if you want to show the pupils extra stuff.
  4. wire247

    wire247 New commenter

    I bought a laser spirit levels for a pound each.
  5. minka1

    minka1 Occasional commenter

    Can i ask any teachers on here how they explain the paradoxes raised by the double slit experiment to their pupils?
  6. www.nuffieldfoundation.org/practical-physics/youngs-slits-light has some good advice, although I would use a laser rather than the bulb. If using the bulb, use one with a vertical filament.

    Also, www.falstad.com/ripple/ is invaluable!
  7. What paradox? Light is a wave and produces constructive and destructive interference, in accordance with an easy to appreciate mathematical construct using path difference and the approximation that waves are plane and parallel for a path greater than 1000wavelengths. (Fraunhofer?)
    However, if you want to consider a particulate explanation, along with considerations of intensity and photon energy, then you are onto interesting territory. However, I would establish double slit phenomenon and confidence in theory and calculations long before introducing wave-particle duality.
    The old Nuffield A level books had pictures of a photo building up "photon-by-photon", taken from an American study in the 1950s - I'm sure they'll be on the internet somewhere.
  8. Damn! Missed the "edit" window!!
    I've just realised you might mean "geometrical optics". I always ask students to predict what will happen when light is projected at 2 slits - they "correctly" predict 2 bright spots!
    Then we look at the evidence and discover the major joy and strength of Physics: you can think what you like BUT if it doesn't fit the observations, something is wrong (cf "what would life be like if Hitler won WW2" etc).
  9. For those who are not familiar with this, you can reduce the light intensity to a point where there is only 1 photon travelling in the system at any time (P=E/t; E=hf). However, you still get an interference pattern, so somehow each photon "knows" about previous and future photons!???
  10. The photon is existing in a state which is a superposition of all possible paths to the screen, including both holes, so the interference pattern is caused by the photon interfering with itself.
    If the photon is observed on its journey, the state of superposition breaks down and no interference pattern is observed
  11. I agree this must be done. However, check out CLEAPSE on using the laser pens bought online. Many have quite high power outputs.
  12. T34

    T34 Established commenter

    Has this assertion been tested?
    Thank you. I shall have a look.

  13. T34

    T34 Established commenter

    Having looked at the reference, I go for strict determinism with hidden variables. Something we don't know about is deciding how much a particular electron deflects.
    I don't like this "goes through two slits at once" stuff. Nor "one electron knows where the other previous electrons went"
  14. God does not play dice eh?
    Well, no-one can explain the mechanism and everyone feels a repulsion to non-determinism when they first study the quantum mechanics. Even Einstein himself. However it it the most scientifically tested and confirmed theory in all of physics. The hidden variables approach allows a bit of comfort though to understandably closed minds.
    We need another genius to see through it all.
  15. I know its not a laser suggestion, but after about 30 minutes of trying to get the interference pattern illustrated with a particularly nervous sixth former we came up with an illustration with two meter rulers. If you hold them slightly apart and draw a wave along both of them you can demonstrate how the path lengths vary as the angle changes. Then if you use the Dr Quantum youtube video


    You may not even need a laser.

    Just a thought.
  16. T34

    T34 Established commenter

    I beg to differ. Non-determinism is the fashion. It is exciting and interesting. It might even allow space for God.
    The theory may describe the end result very accurately. No-one argues against that. But I believe Ptolemy's epicycles described the motion of the planets with considerable accuracy.
    I'm merely cautioning against relinquishing too early the search for mechanism. From our standpoint a lot of fine structure may be overlooked. The unpredictable deflection for individual electrons or photons may be evidence of such fine structure.

  17. minka1

    minka1 Occasional commenter

    With Young's double slit experiment I frankly do not even know what I am trying to understand. Reading the posts on here and the following forum http://www.newton.dep.anl.gov/askasci/phy00/phy00713.htm-
    <hr />

    <hr />
    Hi, I have a question to ask regarding the double slit experiment. In the double
    slit experiment, when we allow just one electron (or in that case, even a
    photon) to pass through the slits (both open), we consider the probability
    functions of the electron and say that the this probability function splits up
    into 2 and after passing through the slits reinforce. That is to say that the
    electrons pass through both the slits simultaneously (provided that no one is
    observing). I find this pretty hard to comprehend especially when I think about
    energy conservation. How is that one electron now appears as two violating the
    Einstein equation E=m*c*c (I have never found any mention of this anywhere else,
    so I understand that my question is inherently absurd but please clear my

    <hr />

    <hr />

    You have hit on one of the most confusing things in quantum
    mechanics. When you are measuring properties of an electron, the electron
    behaves as a particle. It has standard electron mass and other standard
    properties. This only holds at the instant you do the measurement. The more time
    a particle spends without being measured, the more random the electron becomes.
    If you measure which hole the electron passes through, then it will only pass
    through one of the holes. If you do not measure which hole, then it will pass
    through both holes as a wave. This fact is known by measuring the angle of the
    electron's motion after passing through. I do not think any person truly
    understands why this happens. Richard Feynman, perhaps the greatest quantum
    physicists, once said that anyone who claims to understand quantum physics must
    be telling a lie. Accept the fact that nothing tiny is completely a particle or
    completely a wave. Every individual particle or wave has both properties. A
    photon of light tends more toward wave properties. An electron tends more toward
    particle properties. Still, both are somewhere between particle and wave.

    Dr. Ken Mellendorf
    Physics Professor
    Illinois Central College

    <hr />

    It *is* pretty hard to comprehend, but you do not want to be thinking about
    one electron appearing as two electrons. The fact is that you do not know where
    the single electron is with enough accuracy to say which slit it goes through.
    (If you do a measurement to determine which slit the electron goes through, the
    measurement will destroy the interference pattern.) Note that physicists did not
    make this up, and they do not make it happen. All they did was to notice that
    this is the way electrons behave, and then they built a mathematical formalism
    consistent with this behavior, and other known behaviors, so that they could
    guess what an electron might do in different circumstances. The formalism
    describes what electrons do with fantastic precision, but -- as you have noticed
    -- it does not make much intuitive sense, and it is pretty hard to put into

    Tim Mooney

    <hr />

    If you have difficulty understanding the "double slit" experiment, you join
    a large audience of some of the best minds in physics. What makes the experiment
    so frustrating and infuriating is that it is so simple, in principle. It applies
    to electrons as well as photons and presumably to other "particles" as well. To
    the best of my knowledge, there is no interpretation that is generally accepted
    by scientists who know a lot more quantum mechanics than I. All sorts of very
    "strained" explanations have been offered. Here is a recent web page that will
    let you observe the fray, but I do not think it can be called an "explanation".


    Vince Calder

    <hr />

    It is, of course, impossible to really comprehend as quantum mechanics is
    only significant for objects much smaller that we have experience with. It is,
    after all, quite mysterious that when you release a ball, it falls without
    anything visible doing anything to it. However, we have observed that so many
    times and in so many ways that it seems reasonable and even obvious to us.

    It might help if remember (as you correctly and with considerable
    insight point out) that an electron passes through both slits PROVIDED THAT NO
    ONE IS OBSERVING. Actually, the wave function of the electron always spreads out
    throughout the universe, although with very little amplitude where it is very
    unlikely to be. Then when one observes it, the wave function instantaneously
    collapses to the point where the electron is observed. The probability of it
    appearing at any point is proportional to the square of the wave function at
    that point which will, of course, depend on whether the electron wave function
    "went through" two slits. This is the "Copenhagen" interpretation of quantum
    mechanics and how that really works is not terribly well understood and, in
    fact, there is disagreement about it.

    The crucial point is that the
    electron never "appears as two". When observed, it is just one.

    Dick Plano... "
    i think i'm getting to the nub of the question and then it disappears.
    Perhaps that is as it should be.

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