Advanced General Physics 323/324

Modern Physics Unit QM1 Photons: the Particle Nature of Light

Overview: This unit is the first in a series on quantum physics. It begins by discussing the evidence for the particle nature of light: that a beam of electromagnetic radiation can be thought of as consisting of of massless particles called photons as well as having wavelike properties. It then discusses the evidence for energy quantization in atoms which is closely related to the fact that particles such as electrons also have wavelike properties.

Prerequisite: W1


R. Eisberg and R. Resnick Quantum Mechanics of Atoms, Solids, Nuclei and Particles (2nd Ed.) , Chapt. 1 - Thermal Radiation and Planck's Postulate, Sec. 1.2-1.4; Chapt. 2 - Photons--Particle-like Properties of Radiation, Sec. 2.2-2.4; Chapt. 4 - Bohr's Model of the Atom, Sec. 4.4-4.9 and 4.12. The other sections in these chapters may also be of interest to you, but are not required.

Videotape: There is a videotape of a lecture by Prof. Mohan Kalelkar providing an explanation of the key concepts and problem-solving techniques for this unit. If you wish to view the tape during class ask your instructor to set you up in the nearby video room. This tape can also be viewed in the Math and Science Learning Center (MSLC) by asking at the reception desk for Physics 323 Tape QM1 on Photons.

The video may also be viewed online here


This unit is concerned with the experiments that led up to the revolution in physics that we call quantum mechaics. You will learn about two new concepts: that light has a particle as well as a wave nature and that the electrons in a atom can have only discrete ("quantized") energies.

After completing this unit you should understand:

  1. Evidence for photons
    1. The black body radiation spectrum, especially the ultraviolet catastrophe.
    2. The photoelectric effect.
    3. The Compton effect.
  2. Evidence for energy quantization in atoms.
    1. The Rydberg formula for the wave numbers in the hydrogen spectrum (Eq. 4-11).
    2. Bohr's model explaining the Rydberg formula.
    3. The Franck-Hertz experiment in which energy quantization was directly observed.


Chapt. 1: Questions 1,2,8,10,12,13,16 and Problems 1,4,7; Chapt.2 , Questions 1,2,10,12 and Problems 1,2,3,4,9,18; Chapt. 4, Questions 8,9,10,12,13,18,19 and Problems 15,17,19,24.

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