Rutherford Gold Foil Experiment

[alexjlaonnae]

What is the wavelength of the alpha particle Rutherford used in his famous gold foil experiment? How does this compare to the wavelength of the electron?

[Buenchico]

De Broglie to the rescue!

λ = 6.4 X 10^-15 m for Rutherford's alpha particles.
http://electron6.phys.utk.edu/phys250/modules/module%205/nuclear_properties.htm
(Scroll down to 'The Rutherford Experiment)

The wavelength of an electron is dependent upon its rest mass energy and kinetic energy:
http://hyperphysics.phy-astr.gsu.edu/hbase/quantum/debrog2.html
(See the third paragraph for a typical figure)

Good grief! Did I really actually study all this stuff at college? No wonder I chose to teach Maths instead of Physics!
;-)

[ToraToraTora]

correct me if I'm wrong but the helium nucleus would have been on the particle side of dualty here and as such is effectively a machine gun experiement, thus the collisions.

[jim360]

I believe at the time it was viewed as a machine gun experiment, but that doesn't stop you from calculating a particle's de Broglie wavelength. An important aspect of wave-particle duality is that "particles" have "wave-like" properties, and "waves" can be "particle-like".

The speech bubbles are important, I think. The concept of a classical particle, and of a classical wave, should really be discarded entirely at the quantum level.

[jim360]

Oh yes, I meant to add, the alpha particles were viewed as particles, but the gold foil was viewed as having a different structure -- something closer to a thin sheet, rather than the more modern image of mostly empty space filled with tiny atomic nuclei. Thus the surprise was related to the fact that the were collisions at all. Some collisions sent the alpha particles entirely the opposite way round (something that, if memory serves, was almost an accidental discovery, as initially they were only looking for tiny deflections away from a straight line).

[ToraToraTora]

yes absolutely jim, in this case though it suits (most) to visualise what is basically a projectile (or would be at much larger scales) being used to demonstrrate what must have seemed unimaginable even to to Rutherford, namely the almost total lack of matter in matter. An ablsolute classic experiment with, to the masses at least, quite a shocking conclusion.

[jim360]

Oh yes, a classic, and up there with the most important experiments in the History of Science. I'd place it as certainly equal with the Michelson-Morley "there is no ether" experiment.

[Teddio]

Even the highest energy alpha particles, in a head-on trajectory, did not produce a collision with the gold nuclei. The repulsive force on the alpha particle (and on the gold nucleus in the foil) decelerates the alpha particle to a stop at the "radius of closest approach". The force then accelerate the alpha particle in the opposite direction.

[jim360]

Well yes, It's a good point to make. Then again, technically nothing collides ever by a similar sort of argument.

[Teddio]

Good point Jim. A condemned man has nothing to fear from the guillotine blade as the iron atoms will not make contact with his neck.
High energy electrons are even better nuclear-probing particles. They are "points"and can penetrate the nucleus. These probing electrons are relativistic and will interact with the particles in the nucleus by way of their magnetic moment and charge.
Probing high energy protons are equally fascinating.

[jim360]

Well you know what I meant, I'm sure!