Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Cathode Ray Tubes and QM

  1. Feb 1, 2016 #1
    How do CRTs work well and electrons can be sent to exact location on screen in CRT monitors if electrons can behave like wave?

    Is there something in old TVs (for example measurement device) along the road that electron travels to avoid behave like wave?
     
  2. jcsd
  3. Feb 2, 2016 #2

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    The electron gun in a TV set fires very many electrons so what you see on the screen is the average from many quantum-level events.
    The statistics work out so that the average is the classical trajectory.

    The wave nature of electrons is statistical - it is not a wave like you see on water - what gets detected is always a particle in that all the energy arrives in one go at a specific location.

    This means that some electrons from the gun do go astray - it's just that the probability of getting close to the target is so high that the target glows much brighter that you don't see the misses.

    Please view:
    http://www.vega.org.uk/video/subseries/8
     
  4. Feb 2, 2016 #3

    Drakkith

    User Avatar

    Staff: Mentor

    There is nothing that suppresses the wave-like properties of electrons. The key is that the target area, a pixel on the screen, is much, MUCH larger than an electron and it is relatively easy to direct their path with essentially 100% accuracy.
     
  5. Feb 2, 2016 #4
    Hmmm....

    1-) But there is always a chance to find it on another pixel right?

    2-) In double slit experiment, we can observe electrons can diffract and we can't be sure where to to find it on screen. This is also true for without double slit case right? I mean if we fire an electron and even there is no double slit vs. between gun and screen, there is a probability to find electron anywhere on screen. Right?

    Thanks for the answers.
     
  6. Feb 2, 2016 #5

    Svein

    User Avatar
    Science Advisor

    That was a real problem with colour TV CRT's. They had a mask with tightly spaced holes just behind the phosphorous. You had to adjust it as best you could, involving lots of compromises...
     
  7. Feb 2, 2016 #6

    Drakkith

    User Avatar

    Staff: Mentor

    The chances of the electron being found in the next pixel over are far more likely to be because of a malfunction or inaccuracy in the electronics that target the electrons than because of quantum effects.

    Sure, but the chances are absolutely miniscule. They're so small that you don't even need to take the diffraction of electrons into account when designing the TV.
     
  8. Feb 2, 2016 #7

    Drakkith

    User Avatar

    Staff: Mentor

    I doubt any of these TV's were experiencing problems because of electron diffraction.
     
  9. Feb 2, 2016 #8

    Vanadium 50

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    How can baseball pitchers deliver the ball to an exact location in the cater's glove if baseballs are waves?
     
  10. Feb 2, 2016 #9

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    That wasn't a problem of QM. The dimensions of an electron gun (and the whole CRT) take matters way out of the region of diffraction. Any beam spreading and also the resulting lack of colour purity is down to the very tight margins for a shadow mask (and the subsequent improvements) and the fields which deflect the electron beam. Electrons can be treated as 'simple' charged particles when you're designing electron optics in this context, I'm sure.
    Pixels and the spacing of the phosphor dots are not necessarily related. As far as I know, the 'spot' covers several pixels - there are only 700 pixels across the screen and far more colour phosphor triads.
     
  11. Feb 2, 2016 #10

    Svein

    User Avatar
    Science Advisor

    Possibly not, but wave interference was a real problem. TV presenters were not allowed to wear clothes with fine patterns...
     
  12. Feb 2, 2016 #11

    Svein

    User Avatar
    Science Advisor

    deltagun_big.jpg
    https://en.wikipedia.org/wiki/Cathode_ray_tube
     
  13. Feb 2, 2016 #12
    It was due to limitations of PAL and NTSC as well as interlacing jitter
     
  14. Feb 2, 2016 #13

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    OMG. That's a bit of a wild guess; the orders of magnitude are all wrong for diffraction. :wink: It's to do with the colour coding on the signal and it's there on the drive to the tube. In PAL and NTSC, there is a 'subcarrier' at 4.43MHz (PAL), which carries the colour (chrominance) information. This subcarrier forms beats with high frequency video components. These beats are whet you see when the picture content is stripy. A 4.2 MHz grille will produce 230kHz fringes - very visible. The effect is not as visible on single sharp edges so the picture is ok if you avoid regular hf patterns.
     
  15. Feb 2, 2016 #14

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    A detail that the diagram does not show is that the three beams have to come from the same virtual point in space, despite the electron guns and grids are fairly wide, so the shadow mask works onver the whole screen. (Also, the beams go through more than just one hole at a time to make the picture bright enough). The beams from the three guns have to be bent by magnetic fields to achieve this. No wonder those shadow mask tubes had to be set up every day for high quality stdio use. TGF LCDs.
     
  16. Feb 2, 2016 #15

    Drakkith

    User Avatar

    Staff: Mentor

    What does wave interference mean in this context?
     
  17. Feb 2, 2016 #16

    Drakkith

    User Avatar

    Staff: Mentor

    Lots and lots of practice? :-p
     
  18. Feb 2, 2016 #17

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    The shirt problem was intermodulation, whatever he means. It happened on flat screen PAL receivers.
     
  19. Feb 2, 2016 #18

    Svein

    User Avatar
    Science Advisor

    The holes were close enough to create an optical grid...
     
  20. Feb 3, 2016 #19

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    What wavelength would you assign to the electrons, though?
    Hint; look at this link for an idea. It shows the de Broglie Wavelength of a 100eV electron is 0.12nm and electrons in a CRT have many keV of energy.
     
    Last edited: Feb 3, 2016
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Cathode Ray Tubes and QM
  1. Cathode ray tube? (Replies: 8)

  2. Cathode ray tube (Replies: 2)

  3. Cathode ray tube ? (Replies: 2)

  4. Cathode ray tube (Replies: 7)

  5. Cathode Ray Tube (Replies: 11)

Loading...