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Building the CRT's for the Double Slit Experiment

  1. Sep 16, 2012 #1
    Hello, my name is Jordan Heath. I am a UBC third year student in the department of physics and am new to this forum. Me and a couple friends are looking into the possibility of constructing the double slit experiment and are currently wondering if it is possible to salvage a pair of CRT's from old monitors or monochrome TV's. I should say that we do know what we are getting in to and would have access to the UBC machine shop and have a prof. who is experienced in handling CRT's who will guide and prevent us from blowing our selves up. Assuming that we can discharge and de-pressurize the CRT, insert the slits at the correct location, we should have a working demonstration of the double slit experiment. Thats where I am hopping this community can come in, and either lend experience or wisdom on the topic to our project.

    Specifically I have two questions for the community:

    1. Is there any reason why a de pressurized CRT once re pressurized should fail to work?
    2. Is it reasonable to re pressurize a CRT?

    Scientific levels of precision in regards to data collection is not the purpose of this activity, we just want the CRT to display the interference patterns.

    I hope this in the correct location and look forward to hearing your input!

    Jordan Heath
  2. jcsd
  3. Sep 16, 2012 #2


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    Jordan Heath, Welcome to Physics Forums!

    CRTs operate under a vacuum, not pressure. Much of the air in the tube is removed so as to not impede the electrons’ journey from cathode to anode. Once the glass CRT is opened, modified by adding your slits, it must be sealed again.

    The air must be then removed by using a vacuum pump before it can be energised. Additionally, a “getter” should be used to remove more of the remaining air molecules.


    CRTs can be harmful to our health. Be sure every member of your team, including your professor, reads and understands the “Health concerns” section of the Wikipedia website. They include “Ionizing radiation”, “Toxicity”, “Flicker”, “High-frequency audible noise”, “Implosion”, and “Recycling”. See:


  4. Sep 16, 2012 #3


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    Just to be sure: A double-slit experiment with electrons? You know that you will need sub-µm slits and a very good electron source for that? If you want to view the pattern on the screen, you need some additional lenses to magnify it. Will be tricky to pack everything in the space of a CRT tube.
  5. Sep 16, 2012 #4
    In reply to Bobby yes I did mean vacuumed, however lacked the proper vocabulary to express the pressure on the CRT due to that vacuum. Thank you for the well wishes and we will do our reading.

    In reply to mfb, working on the order of micro meters should not be an issue as we have the interest of a professor who has said he should be able to acquire the desired slit and assist us in installing it. Why is the electron source important? We were hoping to spam the slit with electrons, although unscientific we figured the interference pattern should be similar in nature. Just to be clear we are not concerned with sending single electrons through the slit. Getting the pattern on screen is what I personally am most concerned about and doing it in such a way as to not grossly distort things.

    Jordan Heath
  6. Sep 16, 2012 #5


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    The deflection angle is extremely small, so the angles of all the incoming electrons have to be very similar. In other words, the ratio "electron source size"/"electron source to slit distance" has to be very small - something like 10^(-4) or similar. With a distance of ~10cm, this corresponds to ~10 micrometer, which is a very narrow slit itself.
  7. Sep 16, 2012 #6


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    Jordan Heath, This is to discourage you and your team from trying this experiment using a CRT! Two main reasons are the difficulties and the potential dangers. While it is true that wave/particle duality can be demonstrated using photons, electrons, atoms, and even entire molecules, the plan to use electrons by modifying a CRT seems extraordinarily complex and potentially dangerous. Physics Forums prohibits members from contributing to dangerous activities, and yours might be subject to this restriction.

    First, the slits. Thanks, mfb, for educating all of us on some of the requirements for the slits.

    • If you could construct the slits according to mfb’s description, how and where would you mount them?
    • If your first results were not satisfactory, how would you reposition the slits?

    This experiment would clearly require much “trial and error”. Each time you needed to change the slits’ orientation or position, you would need to open the glass seal (release the vacuum), perform the modification, reseal the tube, and pull the vacuum again just to test again.

    Operating Control circuitry complexity.

    Your project requires several power supplies and control circuits:

    • The cathode heater supply-fixed and unchanging.
    • The High Voltage anode supply- fixed and unchanging.
    • Beam deflection control circuits-active (manual) control
    • Beam focusing control- active (manual) control

    Are you, your team, and your professor able to design, test, and build all these?

    Finally, working with CRTS if fraught with dangers, some of which I’ve already pointed out. Also, consider this: the tube is under a high vacuum which stresses the glass walls. One tiny external scratch creates a weak point so that one inadvertent impact could cause an implosion. This causes most of the glass of the tube to rush towards the center. The impacting glass pieces shatter and rebound, sending shards of glass outwards, towards the workers. The phosphor coatings on this glass are toxic to humans.
    See: http://en.wikipedia.org/wiki/Phosphor

    “Phosphors are usually made from a suitable host material with an added activator. The best known type is a copper-activated zinc sulfide and the silver-activated zinc sulfide (zinc sulfide silver).

    The host materials are typically oxides, nitrides and oxynitrides,[10] sulfides, selenides, halides or silicates of zinc, cadmium, manganese, aluminum, silicon, or various rare earth metals. The activators prolong the emission time (afterglow). In turn, other materials (such as nickel) can be used to quench the afterglow and shorten the decay part of the phosphor emission characteristics.”

    So, Jordan Heath, I urge you to reconsider using a CRT to demonstrate that both matter and energy can display characteristics of both waves and particles that demonstrates the fundamentally probabilistic nature of quantum mechanical phenomena.
    See this reference:

    Why not just use a coherent light source such as a laser?


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