Illustrated Quantum Mechanics (English Edition·2nd Edition)

Author: S. Brandt et al.
Publisher:
Publish Date: 1998-03-01
Features:
Fragment:
1. Introduction
The basic fields of classical physics are mechanics and heat on one hand and electromagnetism and optics on the other. Mechanical and heat phenomena involve the motion of particles as governed by Newton's equations. Electromagnetism and optics deal with fields and waves, which are described by Maxwell's equations. In the classical description of particle motion, the position of the particle is exactly determined at any given moment. Wave phenomena, in contrast, are characterized by interference patterns which extend over a certain region in space. The strict separation of particle and wave physics loses its meaning in atomic and subatomic processes. Quantum mechanics goes back to Max Planck's discovery in 1900 that the energy of an oscillator of frequency ν is quantized. That is, the energy emitted or absorbed by an oscillator can take only the values 0, hv, 2hv, .... Only multiples of Planck's quantum of energy are possible. Planck's constant is a fundamental constant of nature, the central one of quantum physics. Often it is preferable to use the angular frequency ω = 2πν of the oscillator and to write Planck's quantum of energy in the form 1.1
The Photoelectric Effect
The photoelectric effect was discovered by Heinrich Hertz in 1887. It was studied in more detail by Wilhelm Hallwachs in 1888 and Philipp Lenard in 1902. We discuss here the quantitative experiment, which was first carried out in 1916 by R. A. Millikan. His apparatus is shown schematically in Figure 1.1a. Monochromatic light of variable frequency falls onto a photocathode in a vacuum tube. Opposite the photocathode there is an anode—we assume cathode and anode to consist of the same metal, which is at a negative voltage U with respect to the cathode. Thus the electric field exerts a repelling force on the electrons of charge -e that leave the cathode. Here e = 1.609 × 10?1? Coulomb is the elementary charge. If the electrons reach the anode, they flow back to the cathode through the external circuit, yielding a measurable current. The kinetic energy of the electrons can therefore be determined by varying the voltage between anode and cathode. The experiment yields the following findings.
1. The electron current sets in, independent of the voltage U, at a frequency ν? that is characteristic for the material of the cathode. There is a current only for ν > ν?.
2. The voltage U? at which the current stops flowing depends linearly on the frequency of the light (Figure 1.1b).