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There are a many alternate light and dark regions present in the cathode ray tube, as the pressure is reduced to minimum, like Crooke's dark space,negative glow, Faraday's dark space and at very low pressures even the positive column is present in the form of striations. I understand the reason for Crookes's dark space that there is very high potential near the cathode, due to which electrons escape faster than the heavy positive ions and there is no release of light as there is no combination of positive ions and electrons, but what are the reasons for other dark and light spaces? And why are they alternate?enter image description here

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  • $\begingroup$ Not being familiar with this phenomena, I can only guess. Could you post some images? $\endgroup$ – dmckee Jul 3 '16 at 16:40
  • $\begingroup$ @dmckee Added the image $\endgroup$ – user456 Jul 3 '16 at 16:44
  • $\begingroup$ I feel like we need a tag to express the idea that the phenomena/experiment under discussion belongs to the hazy bunch of observations that motivated the old quantum mechanics (i.e. the first 25 years of the 20th century). The class for that stuff is often called 'modern physics', but I don't think that's a particular good tag. $\endgroup$ – dmckee Jul 3 '16 at 21:48
  • $\begingroup$ @dmckee LOL, when one thinks that this stuff is 100 years old :) I guess the linguists classify 400 year old Shakespeare as early modern English, so maybe "early modern physics"? Or "early / nascent quantum physics"? $\endgroup$ – WetSavannaAnimal Jul 3 '16 at 23:24
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Regularly spaced light and dark bands far from either anode suggests a simple matter of the electrons picking up the kinetic energy necessary to excite the neutral atoms, giving most of it up when they do so, and then re-accelerating down the tube.

We would expect a couple of diagnostic features:

  • The bands nearer the cathode to be more sharply defined and those far from the cathode to become less and less distinct.
  • The bands would blur and then disappear as the pressure was reduced to the point that the mean free path of the electrons became comparable to the distance over which the excitation energy was obtained starting from rest.

If this is the case it would be closely related to the phenomena observed in the Frank-Hertz experiment.

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    $\begingroup$ Not to promote myself at all, but this phenomena with the very features you describe was visible in an experimental setup at our university. I took a video of it for an SE question: link to Youtube (made with a potato, don't expect too much). The setup is described in the question itself. Indeed, the zones move further and further away from each other as we decrease the pressure (not on video), to the point when there're no zones left at all. $\endgroup$ – Neinstein Dec 17 '18 at 10:15

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