From: Ronald Wong (ronwong@inreach.com)
Date: Tue Aug 31 2004 - 00:45:14 PDT
Message-Id: <l03102803bd55215de9cf@[209.209.19.143]> Date: Tue, 31 Aug 2004 00:45:14 -0700 From: Ronald Wong <ronwong@inreach.com> Subject: Re: Rutherford's gold foil experiment
In Pinhole Digest #1567, Geoff said:
>I just found a cool web site that models Rutherford's famous gold
>foil experiment:
>http://micro.magnet.fsu.edu/electromag/java/rutherford/
>(It's a Java applet.)
>
>While I think this web site is cool, I am also perplexed by part of
>it. As you can see in the applet, the alpha particles only begin to
>be repelled by the gold nuclei when the slit through which they pass
>is opened to a certain amount. This seems to me to be an example of
>diffraction, but why? I would guess that the alpha particles
>diffracting through the lead slit would be totally independent of
>whether some were deflected by the gold nuclei.
Geoff:
Rutherford's experiment involved the scattering of alpha particles off the
nuclei of the atoms of a very thin sheet of gold - around 0.1 micrometer
thick.
Alpha particles are nothing more than the nuclei of helium atoms. So the
experiment involved nothing more than collisions between one atomic nuclei
and another. These are very small particles. Their diameters are of the
order of a hundred trillionth of a meter (okay - 10 femto meters) - with
the alpha particle considerably smaller in size than the gold nuclei. Since
the distance between the nuclei of gold is around 10 000 greater than their
diameter, the chances of hitting them with an alpha particle was pretty
slim (nobody knew this at the time of Rutherford's experiment - that's what
the experiment was about to discover).
If you wanted to diffract alpha particles using a single slit, the opening
would have to be of the order of the particle's size - around 10^-14 meters.
The initial opening in the applet (as well as the slit in the original
experiment) was considerable greater than that - in the case of the applet,
by a factor of ten thousand. So no diffraction was taking place in the
applet - or the experiment. You can see this in the applet by noting that,
as the opening of the slit gets greater, instead of spreading out, the
stream of particles simply becomes broader.
In fact, if this was a case of diffraction, increasing the
opening would have caused any diffraction - any "spreading
out" - to become less.
The broader beam of alpha particles increased the likelihood that one of
the particles would collide with the nucleus of a gold atom.
With the film of gold so thin, a very narrow beam of alpha particles stood
so little chance of hitting one of the gold nuclei that most of them went
right on through the thin sheet of gold as if nothing was there. Because of
this, it was only when the beam width was increased that the incidence of
scattering became more apparent.
In the applet, this begins to occur when the opening is around 2 nm and
becomes even more so as the opening becomes larger for the reason stated
above.
Hope this clarifies things.
ron
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