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TGIF (The Great Inerference Fiasco)
Light as Particles, Matter as Waves
Under constructions.
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. HW#2 is now available. Solutions to HW#1 are also available. |
First 10 minutes quiz: End of class on Friday.
Lab #3 writeups.
The foundations of Atomic Physics:
Side left: Interference & Diffraction.
Or is that exactly so? Presentation: Double slit ("wave breakers") experiment with very very little light: Results: Amazing! With few photons - screen randomly illuminated. More photons: get "interference pattern" (There so called "garbage piles" wherever there are strong waves in the previous class). What's going on? If the light behaves like particles - we should expect only two
piles in front of the
If the light behaves like waves, why do we get the random spots at low illumination? Einstein: The light behaves like both! The photon is interfering with itself! Each photon carries a specific energy. The quantitative relationship between wave behavior and particle behavior: Energy of a light particle (photon) = h x c/l Comes Thompson, 1898, electron IS a particle. Comes lord Rutherford 1911: Nucleus & electrons. . Comes Bohr, 1912 = save from catastrophe, but not rational. DeBroglie, 1919. His suggestion: If light behaves like both particles & waves, why shouldn't particles? At that time the general picture of the atom was that there was a nucleus (found 1911 by Ernst Rutherford, England), and that the electrons orbited the nucleus in certain distances only. For a hydrogen atom that was: 0.05 nm, 0.2nm, 0.45nm, 0.8nm, etc (Niels Bohr, 1912). There were no electrons IN BETWEEN Electrons orbiting a nucleus. DeBroglie asked: Why are there no electrons in between? His answer: Because the electrons behave like waves! (Back to "junk piles" picture). So do they make diffraction patterns? If the circumfrance of the orbit fits an integer number of wave crests - the electron interferes with itself constructively (like in the "garbage piles" on the beach). Otherwise the electron interferes with itself destructively - and there is no electron! (No waves between "piles" on the beach). The levels of an atom have 1 constructive interference for orbit 1, 2 for 2 , etc. Connection between matter (particles) & light (photons): emission from atoms. Whan an electron skips from orbit 2 to orbit 1 a photon is emitted. Question: Given that the energy of orbit 1 is 0 and orbit 3 is 2 x 10-19 Joule, in which part of the spectrum is the emitted photon? Answer: IR. Addendum, Historical Perspective : Historical overview of the connection of today's class to the uncertainty principle by Werner Heisenberg: De-Broglie suggested that the atom's energy levels are
related to
The energy of atomic levels in De-Broglie's model were related
to the "wave length" of that
is the momentum in regular physics). In Einstein's first paper on photons (Same year! 1905) P=h/WL. (That's what Einstein invented in 1905. This relationship later
grew up
INSTEAD OF EQUALITY)
At the time of DeBroglie, 1919, the atom was described with the
Bohr
where r is the distance of the electron from the nucleus). That synthesis led Werner Heisenberg to speculate that the electron-wave
orbits in which the electron may interfer with itself! For that purpose modern quantum mechanics was invented, with the principle which was known, but not fully used or understood, by Einstein in
1905!
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