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Copyright © 2012 Pearson Education Inc. PowerPoint ® Lectures for University Physics, Thirteenth Edition – Hugh D. Young and Roger A. Freedman Chapter 39 Particles Behaving as Waves
39 Lecture Outline Modern Physics University Physics 13th Edition
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Chapter 39 Outline and Powerpoint
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video slideCopyright © 2012 Pearson Education Inc.
PowerPoint® Lectures for University Physics, Thirteenth Edition –
Hugh D. Young and Roger A. Freedman
Chapter 39
Goals for Chapter 39
To study the wave nature of electrons
To examine the evidence for the nuclear model of the atom
To understand the ideas of atomic energy levels and the Bohr model
of the hydrogen atom
To learn the fundamental physics of how lasers operate
To see how the ideas of photons and atomic energy levels explain
the continuous spectrum of light emitted by a blackbody
To see how the Heisenberg uncertainty principle applies to the
behavior of particles
Copyright © 2012 Pearson Education Inc.
Introduction
At the end of the 19th century light was regarded as a wave and
matter as a collection of particles. Just as light was found to
have particle characteristics (photons), matter proved to have wave
characteristics.
The wave nature of matter allows us to use electrons to make images
(such as the one shown here of viruses on a bacterium).
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De Broglie waves
The physicist de Broglie hypothesized that the relationships E = hf
= hc/ and p = h/ for photons also apply to electrons. Thus
electrons should have wave characteristics.
The de Broglie hypothesis was confirmed by the discovery that
electrons undergo diffraction, just like x rays do (see Figure
39.4).
Read Problem-Solving Strategy 39.1.
Follow Example 39.2—Energy of a thermal neutron.
© 2012 Pearson Education, Inc.
Q39.1
In order for a proton to have the same momentum as an
electron,
A. the proton must have a shorter de Broglie wavelength than the
electron.
B. the proton must have a longer de Broglie wavelength than the
electron.
C. the proton must have the same de Broglie wavelength as the
electron.
D. not enough information given to decide
E = hf, p = h/lambda
A39.1
In order for a proton to have the same momentum as an
electron,
A. the proton must have a shorter de Broglie wavelength than the
electron.
B. the proton must have a longer de Broglie wavelength than the
electron.
C. the proton must have the same de Broglie wavelength as the
electron.
D. not enough information given to decide
*
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An electron is accelerated from rest by passing through a voltage
Vba. The final wavelength of the electron is 1.
If the value of Vba is doubled, the wavelength of the accelerated
electron (assumed to be nonrelativistic) changes to
Q39.2
A.
B.
C.
D.
*
© 2012 Pearson Education, Inc.
An electron is accelerated from rest by passing through a voltage
Vba. The final wavelength of the electron is 1.
If the value of Vba is doubled, the wavelength of the accelerated
electron (assumed to be nonrelativistic) changes to
A39.2
A.
B.
C.
D.
*
Electron microscopy
The wave aspect of electrons means that they can be used to form
images, just as light waves can. This is the basic idea of the
electron microscope (see Figure 39.5 at right).
Follow Example 39.3—An electron microscope.
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Atomic line spectra
If a sample of a certain element is sealed in a glass tube and
heated to form a hot gas, the light emitted by atoms in the sample
includes only certain discrete wavelengths. The spectrum of this
light (“line spectrum”) is different for different elements.
Nineteenth-century physics could not explain this.
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The nuclear atom
Rutherford probed the structure of the atom by sending alpha
particles at a thin gold foil. Some alpha particles were scattering
by large angles, leading him to conclude that the atom’s positive
charge is concentrated in a nucleus at its center.
Refer to Figures 39.11 (below) and 39.12 (right) and then follow
Example 39.4.
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The failure of classical physics
Rutherford’s experiment suggested that electrons orbit around the
nucleus like a miniature solar system. However, classical physics
predicts that an orbiting electron would emit electromagnetic
radiation and fall into the nucleus (see Figure 39.14). So
classical physics could not explain why atoms are stable.
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Atomic energy levels
Niels Bohr explained atomic line spectra and the stability of atoms
by postulating that atoms can only be in certain discrete energy
levels. When an atom makes a transition from one energy level to a
lower level, it emits a photon whose energy equals that lost by the
atom (see Figure 39.16 at lower left).
An atom can also absorb a photon, provided the photon energy equals
the difference between two energy levels (see Figure 39.17 at lower
right). Follow Example 39.5.
Insert Figure 39.16
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The Bohr model of hydrogen
Bohr explained the line spectrum of hydrogen (see Figure 39.25
below) with a model in which the single hydrogen electron can only
be in certain definite orbits.
In the nth allowed orbit, the electron has orbital angular momentum
nh/2 (see Figure 39.21 at right). Follow Example 39.6.
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Hydrogen spectrum in more detail
The line spectrum at the bottom of the previous slide is not the
entire spectrum of hydrogen; it’s just the visible-light
portion.
Hydrogen also has series of spectral lines in the infrared and the
ultraviolet. See Figure 39.24 below.
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Hydrogen-like atoms
The Bohr model can be applied to any atom with a single electron.
This includes hydrogen (H) and singly-ionized helium (He+). See
Figure 39.27 below.
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The laser
Atoms spontaneously emit photons of frequency f when they
transition from an excited energy level to a lower level. Excited
atoms can be stimulated to emit coherently if they are illuminated
with light of the same frequency f. This happens in a laser (Light
Amplification by Stimulated Emission of Radiation). See Figure
39.28.
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Continuous spectra and blackbody radiation
A blackbody is an idealized case of a hot, dense object. Figure
39.32 (at right) shows the continuous spectrum produced by a
blackbody at different temperatures.
Classical physics could not explain the shape of the blackbody
spectrum. Planck provided an explanation by assuming that atoms in
the blackbody have evenly-spaced energy levels, and emit photons by
jumping from one energy level down to the next one.
Follow Example 39.7—Light from the sun.
Follow Example 39.8—A slice of sunlight.
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The Heisenberg Uncertainty Principle revisited
The Heisenberg uncertainty principle for momentum and position
applies to electrons and other matter, as does the uncertainty
principle for energy and time. This gives insight into two-slit
interference with electrons (see Figure 39.34 below).
Follow Example 39.9—The uncertainty principle: position and
momentum.
Follow Example 39.10—The uncertainty principle: energy and
time.
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2.
l
1
2.
l
1
2.
l
1
2.
l