I am learning about 'cathode ray discharge tube'. I know when electrons are given out from cathode, they travel to anode, because anode being positively charged attract those electrons and continue to accelerate those electrons. When the electron hit any of the gas atom in tube, it helps to knock out electron from it and therefore, electron knocked also start moving towards anode. But i think that one stage would come when there are no electrons left with gas atoms those can be knocked out and when electron hits the gas ions, it would just slow down the electrons(emitted by cathode) and eventually leading to no ray at the end.

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    $\begingroup$ Electrons do recombine with positive ions rather quickly. Somewhat similar scenario to the one you've described occurs in cathode dark space. Note though that stripping off all electrons (and often even more than just a few) requires tremendous energies and extreme conditions, e.g. deep inside stellar matter. $\endgroup$
    – andselisk
    Commented Jul 28, 2019 at 9:27
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    $\begingroup$ @andselisk. or just a very good vacuum, with just a few hydrogen atoms in the CRT, perhaps <10^-14 torr. Not many atoms to ionize... $\endgroup$ Commented Jul 28, 2019 at 21:10
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    $\begingroup$ A CRT is nowhere near 1E-14 torr. In the 1E-5 or 1E-6 perhaps if lucky. $\endgroup$
    – Jon Custer
    Commented Jul 28, 2019 at 21:21
  • $\begingroup$ Why it involves knocking out and leading to chain. Why not the electrons travel directly from cathode to anode? $\endgroup$ Commented Jul 28, 2019 at 22:08
  • $\begingroup$ The behaviour of electrons in a cathode ray tube depends on the amount of gas in the tube. When there is lots of gas there are lots of collisions and what you observe is very different from when the tube is a hard vacuum and most electrons go straight from cathode to anode. $\endgroup$
    – matt_black
    Commented Jul 29, 2019 at 23:44

1 Answer 1


For a cathode ray tube (old-style TV), you want the electrons traveling collision-free in a straight line from the cathode to the screen (anode), where they generate light hitting a phosphorescent layer. For a neon light, you want the electron hitting gas particles so that the entire tube glows.

This is further explained here: Why does the cathode ray tube only start glowing at low pressures?

  • $\begingroup$ Then for cathode ray tube don't we need any gas? As the path of electrons would remain straight only if they don't hit other gas atoms. Suppose after hitting too they remained in the straight path, then what's the use of gas here as the electrons can directly reach from one end (cathode) to the other end (anode). $\endgroup$ Commented Jul 29, 2019 at 9:39
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    $\begingroup$ Cathode rays are electrons emitted from the cathode. The discharge is from gas collisions. In a TV tube, there's no gas involved, and the electrons zoom past the anode to hit the screen (they are then collected by other anodes). If they are in a tube with gas, they can hit gas molecules and light up the tube as they head toward the anode (no screen). $\endgroup$ Commented Jul 29, 2019 at 13:53

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