AP Night Lecture #5:
Quantum Physics

Online Video Lessons:

For animated lessons with narration and problems, visit HippoCampus's AP Physics B II website and scroll down to the "Atomic Physics and Quantum Effect" topics.

Applet Trouble? If you have trouble running the applet below, visit www.java.com to download and install the latest version of Java onto your computer.
  1. Planck's quantized energy concept
    1. Blackbody radiation not predictable with wave model of light
    2. Planck proposes that radiated energy comes in tiny bundles where E=hf (h = Planck's constant; f = light frequency)
  2. Photoelectric effect
    1. Electrons can be ejected from a piece of metal when it is struck by light
      1. only light above certain frequency (f0) works
      2. this doesn't fit wave model of light
    2. Einstein proposed that all light is quantized into little bundles called photons
    3. Only photons with f > f0 can eject electrons
    4. E=hf0 thus yields minimum energy to eject (called work function Wo or φ)
    5. Any leftover energy becomes kinetic energy (KE = hf - hf0 or KE = hf - φ)
    6. If a voltage V0 (stopping potential) is applied to the plate to stop electrons from being ejected, then because V = W/q, KE = eV0 (q = e = electron charge = 1.60 × 10-19 C)
    7. Photoelectric Effect Java Applet
  3. X-ray production
    1. Photoelectric effect in reverse (fire electrons at metal, which causes emission of high-energy photons called X-rays)
    2. If we assume all of the electron's K is converted into a photon of energy, then K = eV0 = hfmax = (hc)/λmin
  4. Wave and particle duality
    1. Einstein showed light can act like a particle as well as a wave; light particle called a photon
      1. photons have no "rest mass"
      2. photon energy E = hf where h = Planck's constant (given on test) and f = photon's frequency
      3. photon energy can be equalized to mass using Einstein's E=mc2 (so m = hf/c2)

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