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1.One of the observations I learned was that the glass tube begins to glow with a brilliant green light. Many websites I read through refer to a fluorescent material. However, as shown in the above diagram there was no fluorescent material in the experiment carried out first on the cathode ray tube. So where does the green glow come from. Is this the color of the radiation itself?

2."Cathode rays travel in straight lines. That is why, cathode rays cast shadow of any solid object placed in their path. The path cathode rays travel is not affected by the position of the anode." I just can't seem to understand this explanation of the one of the observations.Also, different websites analyses this observation differently. For example, " The cathode rays consist of material particles because they produced shadow of objects placed in the way"

3.Two of the conditions of the experiment were air at very low pressure and secondly a very high potential difference. Could someone please tell me why these conditions were necessary?

I know the questions are very silly but because different websites refer to different things, I am becoming confused with something that should be simple to understand.

  • $\begingroup$ Strictly speaking, this is not a cathode ray tube. It could be called a plasma glow discharge tube or a vacuum diode. A CRT would be similar but would have the anode close to the cathode and it would have a hole in it for the accelerated electrons to pass through. The cathode rays would be the beams of electrons passing beyond the anode. When the sources say they travel in straight lines, they mean once they are far from the acceleration region between the electrodes. $\endgroup$
    – pwf
    Jul 17, 2015 at 6:11

3 Answers 3


The light emitted from the electron stream comes from nitrogen and oxygen molecules. Green is mostly from oxygen. The negative electrons are pulled strongly enough by the positively charged anode that they whack the electrons in the O2 and N2 molecules, putting them into higher energy states or knocking them free of the molecule. In a short time, seconds or a fraction of a second, these electrons rejoin the ionized molecules, fall back into the ground states through one or more quantum decays emitting photons.

Note that the concept of flourescence isn't relevent.

For the electrons emitted by the cathode to do this, they need enough oomph. How fast they move is determined by the strength of the electric field. A stronger field is made by a higher voltage difference applied to the anode and cathode. This is usually much stronger than needed to merely ionize air molecules - we want to those electrons to fly to the anode rather than be scattered by their interactions with the molecules.

To have the electron-molecule interactions, of course we need molecules. That's why the glass tube isn't just a plain vacuum. It can't be air at normal pressure, because that would be too many molecules in the paths of the electrons. The electrons would be deflected and lose a small part of their kinetic energy with each interaction. Normal air suitable for everyday human use would involve so many interactions in a short distance, the electrons be scrambled into a fuzzy cloud of plasma, and not make it to the anode but by an uninteresting, undramatic process of diffusion through the air. You'd see a small area of glow right around the cathode, perhaps, depending on conditions. So, we need a bit of air so something interesting happens, but not enough that it gets in the way.

The electrons travel in straight lines (or nearly straight curves) because the electric field between the anode and cathode is approximately uniform, that is, straight lines. Like a ball falling in gravity, the electrons are on parabolas, but there isn't much transverse velocity, so like a ball falling straight down, the parabolas are just lines. Also, the low-pressure air tends to put a limit on the electrons' speeds, affecting the electrons' motions to more closely follow the electric field.

Nice explanation of molecular excitations at http://www.atoptics.co.uk/highsky/auror3.htm

  • $\begingroup$ This was the answer I was looking for,simple but concise. Just to make sure, the high voltage is just required for the electrons in the cathode to leave with a large enough momentum to whack of electrons but also to reach the anode? Also, the low pressure is just used so that the air particles do not get in the way of the electrons coming out of the cathode? $\endgroup$
    – Eliza
    Dec 21, 2013 at 9:26
  • $\begingroup$ Just accidently came upon this - a cyclotron beam ionizing air, plain open air in the room. So, a charged particle beam doesn't have to be in a near-vacuum to make a visible glow. But then, this is protons or deuterons, and with a lot of energy. en.wikipedia.org/wiki/File:Cyclotron_with_glowing_beam.jpg $\endgroup$
    – DarenW
    Dec 23, 2013 at 3:29
  • $\begingroup$ In the tube the electrons pick up energy slowly and if they get on average much less than the lowest excitation energy before hitting a molecule they will never (or so rarely that it may as well be never) get enough to excite anything. Then you get no glow. In the case of the cyclotron that Daren points at they start with far more energy than is needed to excite (and indeed ionize) the air molecules. $\endgroup$ Dec 23, 2013 at 17:58

Notice the "Air at very low pressure"? That thin air is what glows. And it has to be very thin or it disrupts the "ray" nature of the phenomena. I'm not sure what exactly was glowing in the earliest experiments, but it might well be the $\mathrm{N}_2$.

Later it was normal to put some mercury in the tubes because it works really well. In fact, that is what a florescent light is: a mercury vapor lamp (generally with some phosphors on the glass to get a nicer spectrum).

  • $\begingroup$ Could you please give a bit more detail on why the air glows when in contact with the cathode ray. $\endgroup$
    – Eliza
    Dec 20, 2013 at 4:49
  • 1
    $\begingroup$ Cathode rays are energetic electrons. They can bump into the electrons in atomic or molecular orbitals they can excite them to higher energy states. When they subsequently fall back to low energy states the release photons (i.e. light). $\endgroup$ Dec 20, 2013 at 4:51
  • $\begingroup$ Many websites and books refer to a "green glow". If the above explanation is the origin of the color, then does the glow necessarily have to be green. Shouldn't it depend on how much the electron is excited?Also, "disrupts the ray nature"- do you mean that if there were more air particles, the electrons in the cathode ray collide more frequently disrupting the ray's path? $\endgroup$
    – Eliza
    Dec 20, 2013 at 4:59
  • 1
    $\begingroup$ @Eliza: The quantum nature of this process was one of the magnificent discoveries of the early 20th century. Also look at Einstein's description of the photoelectic effect for understanding of why the color stays green despite increasing energy of the electrons. $\endgroup$
    – DWin
    Dec 20, 2013 at 5:15
  • 1
    $\begingroup$ @Eliza: see en.wikipedia.org/wiki/Gas-discharge_lamp. The colour doesn't have to be green. It depends on the gas used and the pressure. However as mentioned above, for a given gas an pressure the colour is (mostly) independant of the electron intensity and energy. $\endgroup$ Dec 20, 2013 at 7:20

In this experiment discharge glass tube was taken and at both end of glass two metal plate were placed. they were connected with high voltage battery(10000 volt.) the H2 WAS filled at high pressure and at anode end a layer of ZnS WAS placed.

1) At high pressure i.e. 1atm when high voltage current passed then no change seen in the tube. it is because gas is poor conductor of electricity

2) At low pressure i.e. 1torr when high voltage current passed through tube then a greenish glow is seed on ZnS screen. it is because the molecules get ionized and some negatively charge particle travels from cathode plate to anode plate in form of invisible ray called cathode rays>

3) At very low pressure when high voltage current passed thorgh tube then tube became dark. it is because at very low pressure the scattering of light does not take places.

charge/mass ratio of cathode ray is 1.74*10 power 8 c/gram


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