Day 25: Black body radiation, history

  1. A true lecture, complete with powerpoint, about the history leading up to Planck's e=h nu assumption
Problems in class
  • None

I like to spent at least some time on historical background and context, because I think it is important.

Day 26: Black body radiation, theory

  1. Energy of photon field
  2. Two polarizations, helicity, m selection rule
  3. Relativity (wire of charges vs current), no mono poles, longitudinal and time like cancel
  4. Include mu in derivation (done in homework)
  5. Argue mu = 0
  6. Zero point energy
  7. Gives all laws
  8. Use of T4 and lambda-max in astronomy. HR diagram for distance
Problems in class
  • Solar sailing
  • Bolometer, why does it do what it does

Day 27: Crooke's radiometer

  1. Radiation pressure.
Problems in class
  • Predict what happens when a 100W light bulb illuminates the radiometer, find radiation pressure and propose experiment to test predictions.

The class was divided in three groups. The first group spent most time on finding out what the exposed area was. When the vane rotates, it is changing. The second group tried to re-derive radiation pressure in terms of E and B. The last group realized that based on the 100W we can find the power on the vanes. Since the vane is not rotating in ambient light, there must be static friction. This group got more or less to P=Fv, where v is the terminal velocity, and F the dynamic friction. In general, students get stuck on details and do not see the general picture.

This activity needs more thought and more direction. Of course, when we finally do the experiment, the vane goes in the wrong direction, and the explanation why is still in doubt, see wikipedia. There is a temperature difference causing gas to bounce back faster on the dark side, but this also reduces the incoming density.

Personal Tools