Outline of Presentations. Wave Particle Duality and the Quantum (Bohr) Atom (Dr. Steven Blusk) Particle Discoveries in Cosmic Rays and Accelerators (Dr. Tomasz Skwarnicki) Making Sense of it All - The Standard Model (Dr. Marina Artuso)
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Until about 1900, the classical wave theory of light describedmost observed phenomenon.
Energy of wave A2
In the early 20th century, several effects were observed which could not be understood using the wave theory of light.Two of the more influential observations were:1) The Photo-Electric Effect (~1905)
2) The Compton Effect (1923)
What if we try this ?
Vary wavelength, fixed amplitude
Yes, withlow KE
Yes, withhigh KE
Increase energy by increasing amplitude
electrons emitted ?
electrons emitted ?
No electrons were emitteduntil the frequency of the light exceeded a critical frequency, at which point electrons were emitted from the surface!
Light behaving like a particle with E 1/l
(Most energetic photons)
E = hn = hc/l
(Least energetic photons)
Electron comes flying out
In 1924, A. H. Compton performed an experiment where X-rays impinged on matter, and he measured the scattered radiation.
Problem: According to the wave picture of light, the incident X-ray should give up some of its energy to the electron, and emerge with a lower energy (i.e., the amplitude is lower), but should have l2=l1.
It was found that the scattered X-ray did not have the same wavelength ?
Electroninitially atrest (almost)
E2 = hc / l2
Incident X-rayE1 = hc / l1
Compton found that if you treat the photons as if they were particles of zero mass, with energy E=hc/l and momentum p=h/l The collision behaves just as if it were 2 particles colliding !Photon behaves like a particle with energy & momentum as given above!
~4 M photons
~30 M photons
Using a digital camera with manypixels !A given pixel is very, very small gives fine image resolution
The individual spots on thisimage and on the previous oneare the actual results of individual photons striking thepixel array.
Light reflects (or photonsscatter) from a surface and reaches our eye.Our eye/brain forms an image of the object.
But what if we want to “see” smaller things, like inside an atom, or inside the nucleus, or even inside a nucleon ???
Even with a visible light microscope, we are limited to beingable to resolve objects which are at least about:10-6 [m] = 1 [mm] = 1000 [nm] in size.This is because visible light, with a wavelength of ~500 [nm]cannotresolve objects whose size is smaller than it’s wavelength.
“If light can behave like a particle, might particles act like waves”?
Louis de Broglie
The short answer is YES. The explanation lies in the realm of the uncertainty principle & quantum mechanics, Particles, like photons, also have a wavelength given by:
l = h/p = h / mv
That is, the wavelength of a particle depends on its momentum, just like a photon!The main difference is that matter particles have mass, and photons don’t !
This image was taken with a Scanning Electron Microscope (SEM).
These devices can resolve features downto about 1 [nm]. This is about 100 times better than can be done with visible light microscopes!
IMPORTANT POINT HERE:
High energy particles can be used to reveal the structure of matter !
But how do weknow any of this ?
Before ~1900, scientists knew aboutradioactivity.They knew that certain isotopes emittedvarious types of penetrating radiation.
In 1911, Rutherford set out to test this hypothesis.
Around ~1900, the structure of the atom was not known. Common thinking was that it was like a plum-pudding
Calculations, based on the known laws of electricity and magnetism showed that the heavy alpha particles should be only slightly deflected by this “plum-pudding” atom…
Awarded the Nobel Prize in 1908
The calculations suggestedthat a negligible fraction ofthe alpha particles should be scattered by more than90o.
Contrary to expectations, Rutherford found that a significantly large fraction (~1/8000) of the alpha particles “bounced back” in the same direction in which they came…The calculation, based on the plum-pudding model, was that fewer than 1/10,000,000,000 should do this ???
In Rutherford’s words…“It was quite the most incredible event that ever happened to me in my life. It was as if you fired a 15-inch naval shell at a piece of tissue paper and the shell came right back and hit you.”
The atom must have a solid core capable of imparting largeelectric forces onto an incoming (charged) particle.
nucleus, and in doing so, lose energy, until
they spiral into the nucleus.
energy (i.e., as observed in Hydrogen spectra)
Awarded the Nobel Prize in 1922
not radiate energy
Radiation is emitted when an e- jumps from
an outer orbit to an inner orbit and the energydifference is given off as a radiation.
Electronin lowest“allowed”energy level
Electron falls to the lowest energy level
Electrons circle the nucleus
due to the Electric force
Note: There are many more energy levels beyond n=5, they are omitted for simplicity
So, the drop in PE between the 3rd and 1st quantum state is:
Ediff = E1 – E3 = -13.6 – (-1.51) = - 12.09 (eV)
Quantum physics provides the tools to compute the values of E1, E2, E3, etc…The results are:
En = -13.6 / n2
These results DO DEPEND ON THE TYPE OF ATOM OR MOLECULE
The energy difference is given off in the form of EM Radiation.That is, a photon.
42a + 94Be
126 C +10 n
Picked up where Rutherford left off with more
Awarded the Nobel Prize in 1935
he found that the mass of this new object was ~1.15 times that of the proton mass.
Electrons had been know about since ~1900 (J. J. Thomson et al)
Collisions of alpha particles with mattergave us the picture that the atom has adense core at it’s center composed ofprotons & neutrons.
The fundamental units of matter areprotons, neutrons and electrons.
Atomic spectra could be understood fromquantum theory.
Photons acting like particles, well OK…