Download Hewitt/Lyons/Suchocki/Yeh, Conceptual Integrated Science

Conceptual Physics
Fundamentals
Chapter 15:
QUANTUM THEORY
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
This lecture will help you
understand:
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The Photoelectric Effect
Emission Spectra
Absorption Spectra
Fluorescence
Incandescence
Lasers
Wave-Particle Duality
Particles as Waves: Electron Diffraction
Quantum Mechanics
Uncertainty Principle
Correspondence Principle
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The Photoelectric Effect
“I think it is safe to say that no one
understands quantum mechanics.”
—Richard P. Feynman
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The Photoelectric Effect
Quantization
• the idea that the natural world is granular rather
than smoothly continuous
Quantum
• any elemental particle that makes up matter or
carries energy
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The Photoelectric Effect
The photoelectric effect
• A model for how matter radiates
– hypothesized by Max Planck, a German
theoretical physicist in early 1900s
– warm bodies emit radiant energy (light) in
individualized bundles (quanta)
– energy in each quantum is proportional to the
frequency of radiation
• E ~ f, or with Planck’s constant h, E = hf
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The Photoelectric Effect
The photoelectric effect (continued)
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The Photoelectric Effect
The photoelectric effect (continued)
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The Photoelectric Effect
The photoelectric effect
• Einstein’s view on light
– as a stream of particles, bundles of energy (photons)
– photons interact with matter one at a time
– high-energy photons dislodge electrons from certain
metals
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The Photoelectric Effect
CHECK YOUR NEIGHBOR
In the photoelectric effect, the brighter the illuminating light
on a photosensitive surface, the greater the
A.
B.
C.
D.
velocity of ejected electrons.
number of ejected electrons.
both A and B
none of the above
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The Photoelectric Effect
CHECK YOUR ANSWER
In the photoelectric effect, the brighter the illuminating light
on a photosensitive surface, the greater the
A.
B.
C.
D.
velocity of ejected electrons.
number of ejected electrons.
both A and B
none of the above
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The Photoelectric Effect
CHECK YOUR NEIGHBOR
In the photoelectric effect, the higher the frequency of the
illuminating light on a photosensitive surface, the greater
the
A.
B.
C.
D.
velocity of ejected electrons.
number of ejected electrons.
both A and B
none of the above
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley
The Photoelectric Effect
CHECK YOUR ANSWER
In the photoelectric effect, the higher the frequency of the
illuminating light on a photosensitive surface, the greater
the
A.
B.
C.
D.
velocity of ejected electrons.
number of ejected electrons.
both A and B
none of the above
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4
Emission Spectra
• When energy is imparted to an element,
an electron may be boosted to a higher
energy level. The atom is said to be
excited.
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Emission Spectra
• Excitation
• The frequency of an emitted photon ~ energylevel difference in de-exciting.
E = hf
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Emission Spectra
CHECK YOUR NEIGHBOR
Which has less energy per photon?
A.
B.
C.
D.
red light
green light
blue light
all have the same
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Emission Spectra
CHECK YOUR ANSWER
Which has less energy per photon?
A.
B.
C.
D.
red light
green light
blue light
all have the same
Explanation:
In accord with E ~ f, the lowest frequency light has the lowest
energy per photon.
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Emission Spectra
CHECK YOUR NEIGHBOR
Excitation is the process in which
A.
B.
C.
D.
electrons are boosted to higher energy levels in an atom.
atoms are charged with light energy.
atoms are made to shake, rattle, and roll.
none of the above
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Emission Spectra
CHECK YOUR ANSWER
Excitation is the process in which
A.
B.
C.
D.
electrons are boosted to higher energy levels in an atom.
atoms are charged with light energy.
atoms are made to shake, rattle, and roll.
none of the above
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Emission Spectra
Spectroscope
• arrangement of slit, focusing lenses, and prism or
diffraction grating
• to see emission spectrum of light from glowing element
• When an electron is at a higher energy level, atom is excited and
temporarily loses the acquired energy when it returns to a lower
level and emits radiant energy.
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Emission Spectra
Spectral lines
• forms an image of the slit on the screen using a
spectroscope
• each component of color is focused at a definite
position according to frequency
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Emission Spectra
Spectral lines of hydrogen
• more orderly than other elements
• successive lines get closer until the lines merge
• Swedish physicist and mathematician Johannes
Rydberg discovered that the sum of the frequencies of
two lines often equals the frequency of a third line.
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Emission Spectra
Ritz combination principle
• Rydberg’s discovery called the Ritz Combination
Principle:
The spectral lines of any element include frequencies
that are either the sum or the difference of the
frequencies of two other lines.
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Emission Spectra
Absorption spectra
• Atoms in a gas absorb light of the same frequency they
emit.
• A spectroscope can detect “dark” lines in
otherwise continuous spectrum.
an
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Emission Spectra
CHECK YOUR NEIGHBOR
Most of what we know about atoms is gained by
investigating the
A.
B.
C.
D.
masses of elements.
electric charge of elements.
periodic table of the elements.
light they emit.
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Emission Spectra
CHECK YOUR ANSWER
Most of what we know about atoms is gained by
investigating the
A.
B.
C.
D.
masses of elements.
electric charge of elements.
periodic table of the elements.
light they emit.
Explanation:
Light emitted by atoms, their atomic spectra, are considered to be
the fingerprints of atoms.
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Fluorescence
Fluorescence
• Many materials excited by ultraviolet light emit
visible light upon de-excitation.
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Fluorescence
Fluorescent lamps
• UV emitted by excited gas strikes phosphor
material that emits white light.
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Fluorescence
CHECK YOUR NEIGHBOR
An atom that absorbs a photon can then emit one
A.
B.
C.
D.
only at the same energy.
of any energy depending on the situation.
only at a higher energy.
only at the same or lower energy.
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Fluorescence
CHECK YOUR ANSWER
An atom that absorbs a photon can then emit one
A.
B.
C.
D.
only at the same energy.
of any energy depending on the situation.
only at a higher energy.
only at the same or lower energy.
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Incandescence
Incandescence
• The frequency of radiation emitted by a hot body
is proportional to the temperature of the hot
body.
f ~T
• Radiation curve of brightness versus frequency
for emitted light.
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Incandescence
Incandescence (continued)
• Isolated bells ring with a distinct frequency (as
atoms in a gas do).
• Sound from a box of bells crowded
together is discordant (like light
from an incandescent solid).
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Incandescence
CHECK YOUR NEIGHBOR
Which lamp is more efficient for emitting light?
A.
B.
C.
D.
incandescent lamp
fluorescent lamp
both the same for the same wattage
none of the above
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Incandescence
CHECK YOUR ANSWER
Which lamp is more efficient for emitting light?
A.
B.
C.
D.
incandescent lamp
fluorescent lamp
both the same for the same wattage
none of the above
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Lasers
Lasers
• incoherent light (many frequencies and out of
phase)
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Lasers
Lasers (continued)
• monochromatic light out of phase
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Lasers
Lasers (continued)
• coherent light of identical frequencies in phase
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Lasers
Lasers (continued)
• a device that produces a beam of coherent light
• many types and many ranges of light
• not a source of energy
(as is sometimes thought)
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Wave-Particle Duality
Wave-particle duality
• A photon behaves as a particle when emitted by
an atom or absorbed by photographic film or
other detectors.
• But it behaves as a wave in traveling from a
source to the place where it is detected.
• In this sense, light can be both a wave and a
particle!
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Wave-Particle Duality
Wave-particle duality (continued)
• This image is built up photon by photon.
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Wave-Particle Duality
Double-slit experiment
• The light passing through two slits, a, forms an
interference pattern, b, shown graphically in c.
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Particles as Waves: Electron
Diffraction
Particles as waves: electron diffraction
• Every particle of matter is associated with a
corresponding wave. According to Louis de
Broglie, a particle’s wavelength is related to its
momentum.
wavelength =
h
momentum
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Particles as Waves: Electron Diffraction
CHECK YOUR NEIGHBOR
When we speak of de Broglie waves, we’re speaking of the
wave nature of
A.
B.
C.
D.
transverse waves.
longitudinal waves.
particles.
quantum uncertainties.
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Particles as Waves: Electron Diffraction
CHECK YOUR ANSWER
When we speak of de Broglie waves, we’re speaking of the
wave nature of
A.
B.
C.
D.
transverse waves.
longitudinal waves.
particles.
quantum uncertainties.
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Particles as Waves: Electron
Diffraction
Electron diffraction
• Interference patterns of beams of light and
electrons compared
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Particles as Waves: Electron
Diffraction
Electron waves
• Electrons orbiting an atomic nucleus form
standing waves.
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Particles as Waves: Electron
Diffraction
Electron waves
• Hence the discrete energy levels in atoms!
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Quantum Mechanics
• The fundamental equation of quantum
mechanics is Schrödinger’s wave
equation, which is:
(Details of this equation are beyond the
scope of this course.)
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Quantum Mechanics
Quantum Mechanics
• In Schrödinger’s wave equation, the thing that
“waves” is the nonmaterial matter wave
amplitude—a mathematical entity called a wave
function, represented by the symbol ψ (the
Greek letter psi). All the information about the
matter waves is contained in the wave function.
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Quantum Mechanics
• Progression from the Bohr model of the atom to
the modified model with de Broglie waves to the
Schrödinger model.
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Quantum Mechanics
CHECK YOUR NEIGHBOR
As to why electrons orbit in only certain orbits, a compelling
explanation views orbital electrons as
A.
B.
C.
D.
particles that morph into waves.
standing waves.
planetary particles.
quantum particles.
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Quantum Mechanics
CHECK YOUR ANSWER
As to why electrons orbit in only certain orbits, a compelling
explanation views orbital electrons as
A.
B.
C.
D.
particles that morph into waves.
standing waves.
planetary particles.
quantum particles.
Explanation:
Standing waves are stable and close in on themselves in phase.
(See Figure 15.31).
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Uncertainty Principle
Uncertainty principle
• The act of observing something as tiny as an
electron probes the electron and, in so doing,
produces a considerable uncertainty in either its
position or its motion.
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Uncertainty Principle
Uncertainty principle (continued)
• German physicist Werner Heisenberg called
this the uncertainty principle.
• When the uncertainties in measurements of
momentum p and position x for a particle are
multiplied together, the product must be equal to
or greater than Planck’s constant, h, divided by
2π, which is represented as h (called h-bar).
∆p∆x ≥ h
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Uncertainty Principle
Uncertainty principle (continued)
• The ∆ is “uncertainty in measurement of”: ∆p is
uncertainty in measurement of p and ∆x the
uncertainty in position. The product of
uncertainties must be equal to or greater than
(≥) the size of h .
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Uncertainty Principle
Uncertainty principle (continued)
• Applies to uncertainties of measurements of
energy and time. The uncertainty in knowledge
of energy, ∆E, and the duration taken to
measure the energy, ∆t, are related by the
expression: ∆E∆t ≥ h.
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Uncertainty Principle
Uncertainty principle (continued)
• Heisenberg’s uncertainty principle applies only
to quantum mechanics.
• it does not apply to
– uncertainties of macroscopic laboratory
measurements
– a shield of nature’s secrets
– the notion that science is basically uncertain
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Uncertainty Principle
CHECK YOUR ANSWER
To which of these does Heisenberg’s uncertainty principle
apply?
A.
B.
C.
D.
measuring room temperature with a thermometer
momentum and distances of a high-speed bullet
a public opinion survey
none of the above
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Uncertainty Principle
CHECK YOUR ANSWER
To which of these does Heisenberg’s uncertainty principle
apply?
A.
B.
C.
D.
measuring room temperature with a thermometer
momentum and distances of a high-speed bullet
a public opinion survey
none of the above
Explanation:
Heisenberg’s uncertainty principle involves the unavoidable
interaction between nature at the atomic level and the means by
which we probe it.
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Correspondence Principle
Correspondence principle
• The correspondence principle, first stated by
Niels Bohr is:
– If a new theory is valid, it must account for the verified
results of the old theory.
• New theory and old must correspond; that is, they must
overlap and agree in the region where the results of the old
theory have been fully verified.
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Correspondence Principle
CHECK YOUR NEIGHBOR
To which of these does the correspondence principle
apply?
A.
B.
C.
D.
The Schrödinger equation leads to Newton’s equations for orbital
motion of satellites.
The energy of a particle can be expressed as E = mc2.
Diffraction can be explained with either particles or photons.
all of the above
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Correspondence Principle
CHECK YOUR ANSWER
To which of these does the correspondence principle
apply?
A.
B.
C.
D.
The Schrödinger equation leads to Newton’s equations for orbital
motion of satellites.
The energy of a particle can be expressed as E = mc2.
Diffraction can be explained with either particles or photons.
all of the above
Explanation:
Unlike Heisenberg’s uncertainty principle, the correspondence
principle is a general rule. Old and new theory must overlap
where both are valid.
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Complementarity
Complementarity
• Wholeness often means accepting alternate
explanations for natural phenomena.
• Opposite ideas can complement one another
(light can be both a wave and a particle).
• Bohr chose the yin-yang diagram to illustrate
complementarity.
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