Cathode Ray Tubes and QM

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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?
 

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  • #2
Simon Bridge
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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
 
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  • #3
Drakkith
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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.
 
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  • #4
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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.
 
  • #5
Svein
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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.
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...
 
  • #6
Drakkith
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1-) But there is always a chance to find it on another pixel right?

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.

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?

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.
 
  • #7
Drakkith
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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...

I doubt any of these TV's were experiencing problems because of electron diffraction.
 
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  • #8
Vanadium 50
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How do CRTs work well and electrons can be sent to exact location on screen in CRT monitors if electrons can behave like wave?

How can baseball pitchers deliver the ball to an exact location in the cater's glove if baseballs are waves?
 
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  • #9
sophiecentaur
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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...
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.
 
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Svein
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I doubt any of these TV's were experiencing problems because of electron diffraction.
Possibly not, but wave interference was a real problem. TV presenters were not allowed to wear clothes with fine patterns...
 
  • #11
Svein
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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.
deltagun_big.jpg

https://en.wikipedia.org/wiki/Cathode_ray_tube
 
  • #12
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Possibly not, but wave interference was a real problem. TV presenters were not allowed to wear clothes with fine patterns...
It was due to limitations of PAL and NTSC as well as interlacing jitter
 
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sophiecentaur
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Possibly not, but wave interference was a real problem. TV presenters were not allowed to wear clothes with fine patterns...
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.
 
  • #14
sophiecentaur
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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.
 
  • #15
Drakkith
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Possibly not, but wave interference was a real problem. TV presenters were not allowed to wear clothes with fine patterns...

What does wave interference mean in this context?
 
  • #16
Drakkith
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How can baseball pitchers deliver the ball to an exact location in the cater's glove if baseballs are waves?

Lots and lots of practice? :-p
 
  • #17
sophiecentaur
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What does wave interference mean in this context?
The shirt problem was intermodulation, whatever he means. It happened on flat screen PAL receivers.
 
  • #18
Svein
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What does wave interference mean in this context?
The holes were close enough to create an optical grid...
 
  • #19
sophiecentaur
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The holes were close enough to create an optical grid...
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.
 
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