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Cathode ray tube

  1. Jul 17, 2012 #1
    In a video demonstration (not animation), cathode rays appear as a beam of green light moving from the cathode to anode inside the tube. The demonstrator brings a magnet near the tube and the green beam deflects. This I can understand. What I can not understand is that how the electron beam which are invisible are made to appear as a green beam and the beam also beautifully bends. What we have read is the fluorescent glow appears only when it strikes the glass. But how the path of the cathode rays in the gaseous part of the tube glows?

    2.In the paddle wheel demo. when the cathode rays hit the wheel it rotates and moves in the forward direction, but how it moves in the reverse direction?

    3.When a maltese cross is placed in the path of the cathode rays, a dark shadow in the shape of the cross appears on the fluorescent screen.When the cross is folded down, a fluorescent glow in the shape of the cross appears in the same place previously occupied by the shadow. I could understand the previous phenomenon but not the latter.

    If any body gives simple explanation for the above questions, it will be helpful to me.
  2. jcsd
  3. Jul 18, 2012 #2


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    You need a vacuum for the rays, but the vacuum is not perfect - electrons can hit the remaining gas atoms/molecules inside and excite them.

    For 2./3., I think there is some context missing.
  4. Jul 18, 2012 #3
    Last edited by a moderator: Sep 25, 2014
  5. Jul 18, 2012 #4
    But the glowing will decrease. For the cathode rays to be constituted one needs very very less (nearly vacuum) gas pressure. Otherwise some gas atoms can even take electrons. It creates a kind of "viscous drag".

    But with very less gas pressure , this viscous force is reduced to a great extent and the "thermions" which strike the gas atom can ionize them , i.e. knock the valence electron from their orbital.
    Last edited by a moderator: Sep 25, 2014
  6. Jul 18, 2012 #5
    For (3) the explanation is right there on youtube:

    "The after-shadow is because the glass loses some of its fluorescence under bombardment by electrons. When the cross is flipped down the previously shadowed glass is exposed to the electron beam and glows brighter than the surrounding glass. With this tube there isn't any phosphor, just natural glass fluorescence"
  7. Jul 18, 2012 #6
    If the tube is like the one I know, there is a fluorescent stripe inside the tube.
    Some electrons hit this stripe and "trace" the path of the beam.
    Does it look like this one?

  8. Jul 19, 2012 #7
    Thank you for all the replies.
  9. Jul 19, 2012 #8


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    You are not actually looking at a pencil beam in that demo. The beam of electrons is in the form of a horizontal fan and the screen is suspended in the tube at a slight diagonal angle so different bits of the fan strike the screen at different distances along the tube - giving you a trace all along the screen. (Like when you shine a torch obliquely along a wall) This shows how a single pencil beam would behave when you introduce magnetic and electric fields to deflect it. It's a clever idea but can actually confuse people if it's not actually explained.

    btw, the 'maltese cross' tubes that I have seen have the end face coated with a phosphor. It makes the green cross much more visible. Also, as I recollect, the colour you get from electrons passing through low pressure air is Purple / Pink and not Green.
    Last edited: Jul 19, 2012
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