Wacky World of Quantum Physics Part 2 Particles acting like waves

De Broglie’s Idea If waves can act like particles, particles must be able to act like waves in the quantum world. De Broglie Wavelength = Planck’s constant /(mass x velocity) = h /mv For most things this wavelength is too small to be observed Bullet 10-34 m, person 10-37 m (not observable) electron 10-10 m (observable) Question: What was the hypothesis that Louis de Broglie made that won him the nobel prize in 1929? Write the equation used to find the de Broglie wavelength of a body. Why do we not observe the de Broglie wavelengths of everyday objects? Activities: Remind students that yesterday we saw a wave (light) acting like a particle. Today we will see opposite. Discuss de Broglie’s hypothesis and his connection with Einstein. Master’s thesis rejected, sent copy to Einstein who writes back to school administration saying he would suggest that they grant this young man a masters before he makes them look extremely stupid. De Broglie’s is the only master’s thesis to receive a nobel prize. Discuss sizes of de Broglie wavelengths of bullets and people. State that they are too small to be observed. 10 to the 24th smaller than an atom. Then point out that electron’s wavelength is 10 to the -10 which is the exact size of atoms.

Electrons Acting as Waves Double slit experiment Interference pattern is produced on screen whether multiple or single electrons are shot Electron is behaving like a wave If each slit is monitored then electron will behave like a particle Detector must change speed of particle and thus cannot know where it would have gone. Question: What pattern is produced when single or multiple electrons are shot through two slits onto a screen? Is this more consistent with the electron being a wave or particle? What happens if one or both of the slits are monitored with a detector? Activities: Show video on double slit experiment from What the Bleep do you know? Comment on how discussion of electron chosing to be a wave or particle is speculative…electrons do not make decisions rather it is the way the experiment is set up that determines whether electron behaves as a particle or wave. In other words, any detector will need to either slow electron down or deflect it to detect it. In doing so you cannot ever know where it would have gone. Point of double slit experiment is to show that electron can behave as a wave.

Bohr Model of the Atom Protons and neutrons in nucleus surrounded by cloud of electrons in several energy levels. Explained spectral lines as photons emitted when electrons fell from higher to lower energy level Different elements have different lines because of different distances between energy levels. Largest atoms in lower left Smallest atoms in upper right Question: How does the Bohr model account for the spectral lines produced by excited atoms of an element? How does it account for each element having unique lines? On the periodic table where are the largest and smallest atoms in diameter? Activities: Show short segment of Ring of Truth video to remind students how spectral lines are formed and why they are different in each atom. Skip section with 1 line spectra machine. Reiterate that spectral lines are do to electrons falling from a higher energy level to a lower and giving off light. The spectral lines were the evidence of these energy levels. Large falls result in high energy and frequency lines, small falls result in low frequency and energy lines. Discuss how different #s of protons will result in energy levels being at different distances for each atom. Result in each element having unique set of spectral lines. Refer to periodic chart of elements and ask what happens to size of atom as move down a family. How about to the right on a period? Where are the largest and smallest atoms for a naturally occuring element?

Problems with Bohr Model Solved Electron collapsing into nucleus Electron is acting like a wave in the atom Waves have no charge so electron feels no attraction to atom Electron only exists at certain distances from nucleus Electron is wave that exists at locations where constructive interference occurs Locations are where the circumference of the circle = some integer multiple of wavelengths Questions: What were two problems with the Bohr model? How does viewing the electron as a wave solve these problems? Activities: State there were two problems with the Bohr model. First the electron should be attracted by the nucleus and eventually crash into it due to electrostatic attraction. Second, why did electrons only exist at certain distances from the nucleus? Explain that if electron is a wave then it has no charge and thus no attaction to the nucleus. Show transparency of electron waves reinforcing themselves from text and explain. Show Hewitt video of him explaining how energy levels are just locations where the circumference is equal to 1,2,3 or some other whole number of wavelengths for the electron.

So Where are We? Newton’s physics works for everyday objects Einstein’s relativity works for very fast objects Quantum theory works for very small objects But quantum theory and relativity do not work well together => there must be something we don’t understand. Sounds a lot like 1900 when thought physics was almost finished.