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I know its the most basic question, but I do understand that cathode rays are beam of electrons and its not 'light'.

But why can't we see any 'reflected' light from the electron beam i.e if we put (shine) some light on the electron beam then why dont the electrons reflect that light and hence become visible (just like everyday objects which reflect light and become visible)?

Is there any way to directly 'see' electrons in a cathode ray tube?

Is it even possible to 'see' electrons?

Are they an 'invisible concept' who's existence can be confirmed even if they can't be 'seen'?

Qmechanic
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Rohit Shekhawat
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    Note that there is a non-negligible chance that arranging for cathode rays to directly impact one's retina would let a person "see" them, if only briefly. The retina responds to a much wider array of things than just the normally visible spectrum, but the cornea, lens, and other parts of the eye tend to filter them out. I've never seen any reports of anyone testing with cathode rays specifically though. That would be challenging to set up. – Perkins Apr 04 '23 at 21:56
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    Electrons were discovered by the interaction with the residual gas in a cathode ray tube. You find their specific charge or mass by measuring the deflection under a magnetic field, it's a college freshman physics lab experiment. – user71659 Apr 04 '23 at 22:36
  • @Perkins I'd question whether the subject's eyesight (or the subject!) would survive whatever you have to do to get the electrons to penetrate all the way through the eye to the retina. – Hearth Apr 06 '23 at 04:09
  • The classical way to "see" electrons in a cathode ray tube is to watch them on TV. Or rather, the TV picture that's created by electrons energizing phosphor dots in a pattern. – pjc50 Apr 06 '23 at 12:06
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    @Perkins - there are several reasonably well known accidents in various accelerator communities where humans experienced high energy electrons hitting their heads. It generally did not end well for the person, but, yes, visual effects were generally noted. – Jon Custer Apr 06 '23 at 13:48

3 Answers3

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Is it even possible to 'see' electrons?

See recent thread Why can’t we see atoms in an optical microscope? for some of the methods used to visualize atoms. These can be used (and are used) to see the electron clouds.

Regarding seeing electrons optically: electrons have size of about 1 Fermi unit, which is $10^{-15}$ meters, whereas the wave lengths of visible light are hundreds of nanometers long, that is ~$10^{-7}$ meters. We cannot see electrons with light, because our measuring stick is almost a billion times bigger than the thing that we are trying to measure.

Is there any way to directly 'see' electrons in a cathode ray tube?

Electrons in a cathode ray tube are very sparse, and cannot be seen for the reasons described above. A very dense electron cloud however would behave as a metal, i.e., it would reflect light - just like the electrons in a metal do.

Remarks

  • As pointed in the comments by @PM2Ring, the actual size of electrons might be even much smaller than the number given above. See Experimental boundaries for size of electron?
  • Very intense electron beams (either natural or artificially generated) can be indeed seen. The caveat however that it is usually not the scattered light, but the light originating from the atoms ionized via collisions with the electrons in the beam, as e.g., in a fluorescent lamp, where the electrons pass through a Mercury vapour.

The image below is taken from this thread:
enter image description here

Roger V.
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    Comments have been moved to chat; please do not continue the discussion here. Before posting a comment below this one, please review the purposes of comments. Comments that do not request clarification or suggest improvements usually belong as an answer, on [meta], or in [chat]. Comments continuing discussion may be removed. – Buzz Apr 10 '23 at 02:16
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Free electrons are able to interact with photons through Thomson scattering at low (non-relativistic) energies and Compton scattering at high energies. The latter involves high energy photons (x-rays or $\gamma$-rays), but the former can involve photons in the visible spectrum. So you can "see" photons scattered by free electrons if you have the instruments needed to detect them.

The frequency of the interactions depends on the cross-section of the interaction and the density of photons and electrons, so you will probably have more success with a denser electron cloud. As a side but related note, the human eye seems to be able to detect individual photons in darkness, but I don't know how close you can get to that in the conditions of a living room.

Is it even possible to 'see' electrons? Are they an 'invisible concept' [whose] existence can be confirmed even if they can't be 'seen'?

Depends on what you mean by "seen". Electrons can be "seen" in e.g. particle colliders by the way the interact with media they pass through (tracks they leave inside a tracker and the energy deposited in calorimeters). In media, they can also cause Cherenkov radiation if they travel faster than light in that medium.

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Vangi
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  • So for a feel of the scale, could you estimate the brightness of the crossing point of, say, a ray from a $10,\mathrm{mW}$ $532,\mathrm{nm}$ laser pointer and a ray from a typical CRT TV electron gun? – Ruslan Apr 05 '23 at 11:29
  • @Ruslan I can't give an estimate, but the rate (number of interactions per unit time) is given by $R = \mathcal{L}\sigma$, where $\sigma \approx 0.7$ barns is the Thomson cross-section and $\mathcal{L}$ is the instantaneous luminosity. This quantity depends on the density of the electrons and photons in the crossing beams, as well as the angle between them and their shape. You can check this document and in particular eq. (23). You can then approximate the quantities for the laser of interest and typical values of a CRT gun – Vangi Apr 05 '23 at 19:19
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Tiny bits of matter, such as electrons, are rather imperceptible, even to the well-aided eye. Just as air seems to fail to reflect light, we know it is comprised of matter and void; gases—and LOTS of space. The key to visibility is to gather matter together, such that their cumulative density and also cumulative size(not number) is greater than the size of the photobeam desired to be bounced off. Carbon Dioxide gas is essentially invisible, while dry ice and it's "fogging" are easily recognized, since the "dense cloud" has been established. However, I'm not convinced this is the proper answer to the question. Consider perhaps the electrons aren't actually offering much in the way of reflection, while our perception is actually that of the atomic nuclei reflecting light... Cathode rays, however, ARE visible—with the proper receptors, primarily because they contain energy, but, certainly are not perceptible with the limited eyes of man. At any rate, when exploring the vast reaches of microphysics, there always seem to be more questions than we actually have correct answers to. This may be by design, since there is still so much we don't know and so much to be in awe of.

RHH727390
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