Download The photons of red light don`t have sufficient energy to eject

Chapter 28
Quantum
Physics
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PowerPoint® Lectures for
College Physics: A Strategic Approach, Second Edition
28 Quantum Physics
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Slide 28-2
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Slide 28-3
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Slide 28-4
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Slide 28-5
The Photoelectric Effect
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Slide 28-12
Swimming Pool Analogy
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Slide 28-13
The Effect of Voltage Between Anode and
Cathode
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Slide 28-14
Photons: Light Quanta
If the photon has enough
energy (greater than the work
function) an electron is emitted.
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Slide 28-15
Checking Understanding
In the photoelectric effect experiment, why does red light not
cause the emission of an electron though blue light can?
A.
The photons of red light don’t have sufficient energy
to eject an electron.
B. The electric field of the red light oscillates too slowly to
eject an electron.
C. Red light contains fewer photons than blue, not enough
to eject electrons.
D. The red light doesn’t penetrate far enough into the metal
electrode.
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Slide 28-16
Answer
In the photoelectric effect experiment, why does red light not
cause the emission of an electron though blue light can?
A. The photons of red light don’t have sufficient energy
to eject an electron.
B. The electric field of the red light oscillates too slowly to
eject an electron.
C. Red light contains fewer photons than blue, not enough
to eject electrons.
D. The red light doesn’t penetrate far enough into the metal
electrode.
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Slide 28-17
Checking Understanding
Monochromatic light shines on the cathode in a photoelectric
effect experiment, causing the emission of electrons. If the
intensity of the light stays the same but the frequency of the light
shining on the cathode is increased,
A.
B.
there will be more electrons emitted.
the emitted electrons will be moving at a higher
speed.
C. both A and B are true.
D. neither A nor B are true.
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Slide 28-18
Answer
Monochromatic light shines on the cathode in a photoelectric
effect experiment, causing the emission of electrons. If the
intensity of the light stays the same but the frequency of the light
shining on the cathode is increased,
A. there will be more electrons emitted.
B. the emitted electrons will be moving at a higher
speed.
C. both A and B are true.
D. neither A nor B are true.
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Slide 28-19
Checking Understanding
Monochromatic light shines on the cathode in a photoelectric
effect experiment, causing the emission of electrons. If the
frequency of the light stays the same but the intensity of the light
shining on the cathode is increased,
A.
B.
C.
D.
there will be more electrons emitted.
the emitted electrons will be moving at a higher speed.
both A and B are true.
neither A nor B are true.
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Slide 28-20
Answer
Monochromatic light shines on the cathode in a photoelectric
effect experiment, causing the emission of electrons. If the
frequency of the light stays the same but the intensity of the light
shining on the cathode is increased,
A.
B.
C.
D.
there will be more electrons emitted.
the emitted electrons will be moving at a higher speed.
both A and B are true.
neither A nor B are true.
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Slide 28-21
Example Problem
Exposure to ultraviolet can damage the skin, as anyone who has
spent too much time in the sun knows all too well. For this
reason, there are suggested limits for exposure to ultraviolet in
work settings. These limits are wavelength-dependent. At a
wavelength of 313 nm, the maximum suggested total exposure
is 500 mJ per cm2 of skin; for 280 nm, the limit falls to 3.4 mJ per
cm2 of skin.
A. What is the photon energy corresponding to each of
these wavelengths?
B. How many total photons does each of these exposures
correspond to?
C. Clearly, the shorter wavelength photons have a much
higher probability of causing damage. Explain why you
might expect this to be true.
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Slide 28-22
X-ray Diffraction: The Bragg Condition
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Slide 28-23
Diffraction Patterns for X Rays, Electrons, and
Neutrons
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Slide 28-24
Matter Waves
An electron beam
passing through a
double slit produces
an interference pattern
similar to that for light.
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Slide 28-25
The Particle in a Box
The possible modes are reminiscent of those for
the modes of a stretched string. The possible
modes have different energies:
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Slide 28-26
Example Problem
Electrons in molecules with long chains of carbon atoms can move
freely along the chain. Depending on what atoms are at the end of
the chain, the electrons may well be constrained to stay in the
chain and not go beyond. This means that an electron will work as
a true particle in a box—it can exist between the fixed ends of the
box but not beyond. The particle in a box model can be used to
predict the energy levels. Such molecules will show strong
absorption for photon energies corresponding to transitions
between energy levels. One particular molecule has a “box” of
length 1.1 nm. What is the longest wavelength photon that can
excite a transition?
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Slide 28-27
Energy Levels and Quantum Jumps
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Slide 28-28
Checking Understanding
What is the maximum photon energy
that could be emitted by the quantum
system with the energy level diagram
shown below? The minimum photon
energy?
A.
B.
C.
D.
E.
F.
G.
7.0 eV
6.0 eV
5.0 eV
4.0 eV
3.0 eV
2.0 eV
1.0 eV
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Slide 28-29
Answer
What is the maximum photon energy
that could be emitted by the quantum
system with the energy level diagram
shown below? The minimum photon
energy?
A.
B.
C.
D.
E.
F.
G.
7.0 eV
6.0 eV
5.0 eV
4.0 eV
3.0 eV
2.0 eV
1.0 eV
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Slide 28-30
Conceptual Example Problem: The Uncertainty
Principle
A neutron beam has an even smaller wavelength than an electron beam.
Why don’t we use neutron microscopes instead of electron microscopes?
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Slide 28-31
Wave-Particle Duality
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Slide 28-32
The Dual Nature of a Buckyball
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Slide 28-33
Summary
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Slide 28-34
Summary
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Slide 28-35
Summary
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Slide 28-36
Additional Questions
Light falling on a photoelectric cell is causing a steady current in
the cell. A filter that transmits red light is placed in front of the
cell, and the current suddenly ceases. This is because
A. the filter has reduced the intensity of the light.
B. the filter has eliminated the highest-energy photons.
C. the filter has reduced the energy of the photons in the
beam.
D. the filter has slowed down the photons in the beam.
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Slide 28-37
Answer
Light falling on a photoelectric cell is causing a steady current in
the cell. A filter that transmits red light is placed in front of the
cell, and the current suddenly ceases. This is because
A. the filter has reduced the intensity of the light.
B. the filter has eliminated the highest-energy photons.
C. the filter has reduced the energy of the photons in the
beam.
D. the filter has slowed down the photons in the beam.
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Slide 28-38
Additional Questions
Which of the following phenomena is best explained by treating
light as a wave?
A.
B.
The threshold frequency in the photoelectric effect
The emission of only certain wavelengths of light by an
excited gas
C. The limited resolution of a light microscope
D. The quantization of energy levels for a particle in a box
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Slide 28-39
Answer
Which of the following phenomena is best explained by treating
light as a wave?
A.
B.
The threshold frequency in the photoelectric effect
The emission of only certain wavelengths of light by an
excited gas
C. The limited resolution of a light microscope
D. The quantization of energy levels for a particle in a box
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Slide 28-40
Additional Questions
Which of the following phenomena is best explained by treating
light as a particle?
A.
B.
The limited resolution of a light microscope
The diffraction pattern that results when x rays
illuminate a crystal
C. The threshold frequency in the photoelectric effect
D. The quantization of energy levels for a particle in a box
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Slide 28-41
Answer
Which of the following phenomena is best explained by treating
light as a particle?
A.
B.
The limited resolution of a light microscope
The diffraction pattern that results when x rays
illuminate a crystal
C. The threshold frequency in the photoelectric effect
D. The quantization of energy levels for a particle in a box
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Slide 28-42
Additional Example Problem
Port-wine birthmarks can be removed by exposure to 585 nm
laser light. Pulses are strongly absorbed by oxyhemoglobin in the
capillaries in the birthmark, destroying them. A typical laser pulse
lasts for 1.5 ms, and contains an energy of 7.0 J.
a.
b.
What is the power of the laser pulse?
How many photons are in each pulse?
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Slide 28-43