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 do they alternate?

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  • $\begingroup$ 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. $\endgroup$ 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"? $\endgroup$ Jul 3 '16 at 23:24

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.

  • 2
    $\begingroup$ 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

If I understand your question correctly, you are asking about the alternating regions of light and dark spaces in the positive column: those are called striations, see, for example [1-3].

As was in principle already said by @dmckee [4], they can be interpreted as locally different ionization rates, with high ionization in the bright areas (because high ionization means also high recombination which results in light being emitted), low in the dark areas. Different ionization rates indicate, at the low pressures that we have here, different energies of the electrons.

The striations are, in my understanding, the result of a typical plasma physics phenomenon: shielding charges immersed into the plasma. If we have a region of enhanced ionization rate somewhere, it means ions are produced there. This can lead, locally, to a positive charge cloud (space charge might be a more common expression), which is shielded by a negative space charge of electrons. This, in turn, reduces the effective accelerating potential the electrons “sees”, thus slowing down the electrons to an energy where they can no longer ionize, therefore the dark area. Getting closer to the anode, they can be accelerated again. And so on.


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