# Modern Physics Learning Goals Fa13

The third unit of Physics 3 deals with modern physics. I was in a rush to get these learning goals put together and only got around to writing content learning goals this semester.

I struggled with what to cover in this unit. I decided to only focus on concepts that we could derive or justify ourselves. We made good connections with earlier concepts like standing waves and thin-film interference but we were light on the applications. I definitely need to think about the goals for this unit over winter break.

**Content Learning Goals**

1. Explain the setup and experimental result of the Michelson-Morley experiment. The student can show (mathematically) how length contraction arises as an explanation of the experimental result.

2. Show (mathematically) how time dilation arises from the requirement that an observer in a moving frame and an observer in a stationary frame both measure the same speed for the speed of light.

3. Explain the apparent paradox in the “Twin Paradox” and explain how the apparent paradox is resolved.

**** We also discussed the “Pole in the Barn Paradox” which should maybe be added to the learning goals.*

4. Describe an experimental result that suggests that it is sometimes useful/necessary to think of particles as being waves.

5. Use the de Broglie wavelength equation to calculate the wavelength of an object or to calculate the momentum of a photon.

6. Explain the conceptual meaning of the position-momentum uncertainty principle (don’t worry about the energy-time version) and understand how the uncertainty principle relates to describing a “particle” as a wave packet.

7. Sketch the eigenstates (i.e. standing waves) for a particle in a 1D box. The student can also use these sketches, together with the de Broglie wavelength equation, to determine the energy of each eigenstate.

8. Explain the physical meaning of the energy of the lowest energy standing wave in a 1D box and discuss how this differs from our classical (i.e. non-quantum) expectations.

9. Relate the wavefunction for a particle to the probability of finding the particle in a certain region of space.

10. Qualitatively explain when a “particle” has a very large or small probability to reflect from a potential barrier or a potential well and make connections between this phenomena and the phenomena of thin-film interference.

**** This should say quantitatively not qualitatively. We calculate well/barrier thicknesses and particle energies that will maximize or minimize reflection probability we just don’t calculate the numerical probability.*

11. Explain what it means to say that atoms have quantized energy levels.