Why are cathode rays invisible?

Question

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'?

Answer 1

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:

Answer 2

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.

Answer 3

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.