So I just read that when alpha particle hit the gold foil sparks were created. And these flashes were used to determine the angle of scattering. So were the sparks created because the alpha particles (positive) excited the electrons in the gold foil? And is a spark just that? Light emitted due to Excitation of electrons and them coming back to a lower energy ( de-excitation)?

Also were any electron even excited out of the gold foil?


2 Answers 2


So I just read that when alpha particle hit the gold foil sparks were created. And these flashes were used to determine the angle of scattering

I think you need to read the text again.

A schematic of the apparatus is as follows.

enter image description here

The alpha particles after they emerged from the gold foil produced flashes of light (you have called the sparks) on a fluorescent screen which were observed using a microscope focussed on the screen.
The flashes of light were produced by the interaction of the alpha particles and the zinc sulphide which coated the screen. The number of alphas hitting the screen per unit time was measured as a function of $\theta$, the angle trough which the alpha particles had been deflected.

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    $\begingroup$ Thank you! The diagram in my book was incomplete and hence it confused me $\endgroup$ May 16 at 8:31
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    $\begingroup$ what is the difference between a spark and flash in physics? $\endgroup$ May 16 at 19:49
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    $\begingroup$ @RoselynnSprinkles Scintillation certainly involves electrons, but not the flow of electrical current. The passing charged particle excites electrons in the material and when they drop back down to their original states photons are emitted. A spark is an electrical discharge where a large number of electrons travel a significant distance as an electrical current through vacuum or an insulating medium experiencing a dielectric breakdown $\endgroup$
    – uhoh
    May 16 at 20:56
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    $\begingroup$ @uhoh And to make things even more confusing, sometimes sparks are actual glowing hot specks of metal which have detached from a parent substrate. $\endgroup$
    – Michael
    May 16 at 21:03
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    $\begingroup$ @RoselynnSprinkles the atoms in the path get excited and make photons. You can't see electrons in a vacuum tube. $\endgroup$
    – user253751
    May 17 at 11:10

The flashes were not produced by the gold foil. The alpha particles were detected using a sheet of a phosphorescent material. Googling suggests this was zinc sulphide, which is certainly a common phosphor used in such experiments.

The alpha particles hitting the zinc sulphide screen do indeed excite electrons, and those electrons then emit light as they decay back to the ground state.

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    $\begingroup$ But would the alpha particles excite the electrons of Gold foil too? And if not then why? $\endgroup$ May 16 at 8:32
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    $\begingroup$ @RoselynnSprinkles Generating light by exciting electrons in solids is actually quite hard because the electrons typically just convert their energy to heat i.e. no light is produced and instead the energy just heats up the solid. So it's very unlikely you'd see light from the gold foil. There are relatively few solid materials (like zinc sulphide) that will produce light when their electrons are excited. This is called fluorescence or phosphorescence. $\endgroup$ May 16 at 8:40
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    $\begingroup$ So what is so different about zinc sulphide that lets it produce light but not other materials? $\endgroup$ May 16 at 8:45
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    $\begingroup$ @RoselynnSprinkles See Why don't phosphorescent molecules just immediately re-emit the original UV photon? $\endgroup$ May 16 at 8:48

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