How big are photons compared to electrons?

[rcgldr]

In the case of a beam of electrons, most of them will flow through a thin surface like foil without any collisions (only a small percentage of the electrons end up colliding with the molecules of the foil), while in the case of a light beam, very few photons will pass through.

Somewhat puzzling given this is the fact what the photons are interacting with is the electrons of the molecules in the foil.

 

[Defennder]

I don't see what this has got to do with size. The photons are absorbed when they hit the foil aren't they?

 

[gendou2]

The classical notion of "size" can be misleading in the subatomic world.
In modern physics, the electron has zero size because it is a point particle.
Similarly, photons don't have a size so to speak, but they do have a wavelength.

http://en.wikipedia.org/wiki/Electron_radius http://en.wikipedia.org/wiki/Photoelectric_effect

 

[HallsofIvy]

One of the key points Quantum Physics, and the reason why we have to think of things as both particles and waves, is that the geometric concepts such as "size" simply become meaningless in the subatomic domain.

 

[Astronuc]

In the case of a beam of electrons, most of them will flow through a thin surface like foil without any collisions (only a small percentage of the electrons end up colliding with the molecules of the foil), while in the case of a light beam, very few photons will pass through.

Somewhat puzzling given this is the fact what the photons are interacting with is the electrons of the molecules in the foil.

Is one comparing visible light with electrons of similar energy, i.e. eV range?

I think one will find gamma rays will pass right through a foil. X-rays have wavelengths on the order of atomic lattices, and that is how one obtains Bragg (X-ray) diffraction.

High energy electrons keV or MeV will mostly pass through a thin foil - and attenuation is a function of electron energy and thickness of the foil.

 

[rcgldr]

Is one comparing visible light with electrons of similar energy, i.e. eV range?

I wasn't taking the energy levels into account, not realising this this would affect the "size" or whatever it is that allows high energy or high velocity sub-atomic particles pass through solids. I do recall that certain types of radiation and/or sub-atomic particles are an issue for space craft. I also recall that some x-ray machines use an electron beam striking a metal shield to generate x-rays.