Physics - Chapter 38
The Atom and the Quantum
Terms & Objectives

New Terms in Chapter 38:

Chapter 38 Core Ideas:

• Background
  • By the beginning of the 20th century, it was commonly accepted that light was a wave (an electromagnetic wave, actually).
    • Evidence:
      • Light displays interference patterns characteristic of waves.
        • Types of interference
          • Waves align and add together for constructive interference.
          • Waves cancel for destructive interference.
    • The speed of light in water matched the prediction for waves, not particles.
    • Maxwell's Equations established light as an electromagnetic wave.
• The Quantum
  • The German physicist Max Planck introduced the idea of the quantum - an energy "bundle" of a particular size - to explain the emission and absorption of light by hot objects.
  • The energy of a quantum of light energy depends on its frequency: E = hf, where h is called Planck's constant - a VERY small number.
  • Planck showed that if matter emits and absorbs energy only in quantum packets, the emission and absorption of light by hot objects could be explained, but Planck had no explanation as to why light waves would be absorbed/emitted in packets.
• The Photoelectric Effect
  • When light shines on certain metals, electrons are ejected from the metal.
  • Brighter light generally ejects more electrons than dimmer light.
  • Higher frequency light generally ejects faster (more energetic) electrons than lower frequency light.
  • There is a "threshold frequency," which depends on the type of metal, so that light of a lower frequency will not eject any electrons, no matter how bright the light.
  • Einstein's explanation (1905):
    • Light is made of particles, called photons.
    • A photon is a quantum of light, so the energy of a photon depends on its frequency (E = hf).
    • An electron is ejected from the metal when a single photon transfers its energy to a single electron.
    • Brighter light means more photons, so more electrons are ejected by brighter light.
    • Higher frequency light means more energetic photons, so more energetic electrons are ejected by higher frequency light.
    • For light below the threshold frequency, photons do not have enough energy to eject an electron.
• Particles as waves
  • Louis De Broglie proposed that all particles have wave properties.
  • The wavelength of a particle is inversely proportional to its momentum.
  • The wavelengths of people-sized particles is much too small to be noticed or detected.
  • The wave nature of electrons can be demonstrated
    • Electrons can produce interference patterns.
    • Technology: the electron microscope.
• The Bohr Model of the Atom
• The wave properties of electrons determine the possible orbits of electrons in the atom.
• An orbit is allowed where the electron waves interfere constructively.
• When an electron "jumps" between orbits, the energy difference is emitted/absorbed as a photon of the corresponding frequency (E = hf).
• Relative Sizes of Atoms
  • An atom with 5 times as many protons and electrons is not generally 5 times larger, since the additional protons exert more force on the electrons and pull them closer to the nucleus.
• Quantum Physics/Quantum Mechanics
  • Quantum mechanics is based on probabilities - which Einstein could not accept.
• Schroedinger Wave Function - gives the probability of finding a particle at a particular location.
  • When an observation is made, the wave function "collapses."
  • The Copenhagen Interpretation says, basically, that quantum mechanics is a complete description of quantum events, which can only be interpreted by their macroscopic (people-sized) effects.
• Heisenbery Uncertainty Principle - you can't measure position/momentum or energy/time to arbitrary precision.

Chapter 38 Objectives:

When you have completed Chapter 38, you should be able to:

1. ... describe the photoelectric effect, and tell how Einstein explained it.
2. ... tell what it means for a quantity to be quantized, and give examples.
3. ... give examples of models of light.
4. ... describe constructive and destructive interference of light.
5. ... describe evidence for the wave nature of light.
6. ... describe evidence for the particle nature of light.
7. ... discuss the relative sizes of atoms.
8. ... describe evidence for the wave nature of electrons.
9. ... discuss the wave properties of matter.
10. ... tell what quantum mechanics is and discuss some of its characteristics.

Possible Misconceptions to Correct:

Do you believe that any of the following statements are true? They AREN'T! When you finish Chapter 38, you should understand that each of these statements is FALSE, and WHY.

• WRONG: If something is a wave it can't be a particle.
• WRONG: If something is a particle, it can't be a wave.

Last update April 6, 2008 by JL Stanbrough