Great Experiments (1) : Rutherford Scattering 偉大的實驗（一）： 拉塞福散射
Hello everybody! Today, I would like to present to you one of the most important experiments in the history of mankind, performed by possibly the best experimentalist ever lived: Rutherford Scattering. The experiment got its name because it was performed by Ernest Rutherford (and his students) in 1911. Ernest had already been awarded a Nobel Prize in Chemistry in 1908 when he performed his famous scattering experiment. Really, I feel he should have been awarded another one in Physics! Okay, so what was the experiment?
(Rutherford was Director of the Cavendish Laboratory at the University of Cambridge, succeeding J.J. Thomson who discovered the electron in 1897.)
The idea of the experiment 實驗的概念
The set-up of the experiment is illustrated in the figure above. Essentially, we have a source of alpha particles (It isn't important to know what alpha particles are exactly, but to know that they are small and are positively charged. For those who are interested, an alpha particle is a Helium nucleus), which shoots alpha particles at a gold foil. Behind the gold foil is a small fluorescent screen that flashes when it is hit by an alpha particle. The small fluorescent screen can be moved around, so that we know where the alpha particles have gone after hitting the gold foil. The scattering angle that we can detect ranges from 0 to 180.
The result of the experiment 實驗的結果
One might think: surely if alpha particles will get stopped by the gold foil just like how if we can't run through a wall? This might be what Rutherford was thinking as well, but Nature likes to surprise us: Most of the alpha particles went straight through. You might think: Wow, what's going on there? The interesting is yet to come: Some of the alpha particles (1 in 8000) were backscattered, meaning that they basically bounced back to where they came from.
Rutherford was so shocked by the results that he reacted with what is now a famous quote: "It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you!"
If you don't know what a 15-inch shell look like, here it is:
Rutherford's interpretation 拉塞福的結論
So how did Rutherford make sense of his bizarre results? His resolution was: (People were aware that things were made up of atoms in those days, so a gold foil would just be a sheet of gold atoms. What they didn't know was what the atom looked like)
- Most of an atom is empty space, hence most of the alpha particles could go straight through the gold foil.
- The atom has a very dense and positively charged region that is responsible for the backscattering: what we now know as the nucleus of an atom.
Rutherford's findings directly contributed to our understanding of atomic structure today. Today, we know that the nucleus of an atom is made of protons and neutrons. Protons are positive while neutrons are chargeless, so the nucleus overall is positively charged.
In the picture above, the red balls are protons, while the blue balls are neutrons. The grey balls are electrons (discovered by J.J. Thomson); they are negatively charged. In an atom, the number of electrons and protons are always the same so that overall the atom is electrically neutral.
If you have patiently read all of the above, congratulations! You now know how us humans know what the atom looks like! And of course, thank you very much for reading and I hope you have enjoyed this interesting (true) story. I am hoping this will develop into a series (Hence this is called Great Experiments (1)). I am also thinking of starting other series, perhaps some Quantum Mechanics or Relativity? Or Astronomy? Any suggestions are welcome!
he was a great scientist. I read about him in college
He certainly was! One of the best!
Yes, he was the one of the best!
We are mostly empty space, confronting thought, isn't it? ;)
Indeed, but it is true!
Great post. I might have some data from college when I did this experiment myself. Kep them coming.
There is also a good youtube video showing a reconstruction of the original experiment.
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This is a great grandfather I admire your business