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Conceptual Physics
11th Edition
Chapter 31:
LIGHT QUANTA
© 2010 Pearson Education, Inc.
This lecture will help you understand:
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•
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Birth of Quantum Theory
Quantization and Planck’s Constant
Photoelectric Effect
Wave–Particle Duality
Double-Slit Experiment
Particles as Waves: Electron Diffraction
Uncertainty Principle
Complementarity
© 2010 Pearson Education, Inc.
Birth of Quantum Theory
• There has been a long historical debate about the
nature of light:
– Some believed it to be particle-like.
– Others believed it to be wavelike.
• Young’s double-slit experiment in 1801 proved
that light was a wave.
• Max Planck in 1900 hypothesized that radiant
energy was emitted in discrete bundles, each of
which he called a quantum.
© 2010 Pearson Education, Inc.
Quantization and Planck’s Constant
• Quantum physics states that in the microworld of
the atom, the amount of energy in any system is
quantized—not all values of energy are possible.
– Example: The energy in a beam of laser light, which is a
whole-number multiple of a single lowest value of
energy—one quantum
• The quanta of light, and of electromagnetic
radiation in general, are the photons.
• Energy of a quanta:
– E = hf
where h is Planck’s constant
© 2010 Pearson Education, Inc.
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
© 2010 Pearson Education, Inc.
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
© 2010 Pearson Education, Inc.
The Photoelectric Effect
• Light shining on the negatively charged, photosensitive metal
surface liberates electrons.
• The liberated electrons are attracted to the positive plate and
produce a measurable current.
• If we instead charge this plate with just enough negative
charge that it repels electrons, the current can be stopped.
• We can then calculate the energies of the ejected electrons
from the easily measured potential difference between the
plates.
© 2010 Pearson Education, Inc.
The Photoelectric Effect
The photoelectric effect (continued)
© 2010 Pearson Education, Inc.
The Photoelectric Effect
The photoelectric effect (continued)
© 2010 Pearson Education, Inc.
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.
© 2010 Pearson Education, Inc.
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.
© 2010 Pearson Education, Inc.
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.
© 2010 Pearson Education, Inc.
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.
© 2010 Pearson Education, Inc.
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.
© 2010 Pearson Education, Inc.
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!
© 2010 Pearson Education, Inc.
Wave–Particle Duality
Wave–particle duality (continued)
• This image is built up photon by photon.
© 2010 Pearson Education, Inc.
Double-Slit Experiment
Double-slit experiment
• Monochromatic light passing through two slits, a,
forms an interference pattern, b, shown
graphically in c.
© 2010 Pearson Education, Inc.
Double-Slit Experiment
• Suppose we dim our light source so that, in effect,
only one photon at a time reaches the barrier with
the thin slits.
• If film behind the barrier is exposed to the light for
a very short time, the film gets exposed as shown
below.
– Each spot represents the place where the film has been
exposed by a photon.
– If the light is allowed to expose the film for a longer time,
a pattern of fringes begins to emerge
© 2010 Pearson Education, Inc.
Double-Slit Experiment
• If we cover one slit so that photons striking the
photographic film can pass only through a single
slit, the tiny spots on the film accumulate to form a
single-slit diffraction pattern.
• We find that photons hit the film at places they
would not hit if both slits were open.
© 2010 Pearson Education, Inc.
Double-Slit Experiment
How do photons traveling through one slit “know”
that the other slit is open and avoid certain regions,
proceeding only to areas that will ultimately fill to
form an interference pattern?
• Each single photon has wave properties as well as particle
properties.
• The photon displays different aspects at different times.
• A photon behaves as a particle when it is being emitted by
an atom or absorbed by photographic film or other
detectors, and behaves as a wave in traveling from a
source to the place where it is detected.
• So the photon strikes the film as a particle but travels to its
position as a wave that interferes constructively.
© 2010 Pearson Education, Inc.
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 
© 2010 Pearson Education, Inc.
h
momentum
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.
© 2010 Pearson Education, Inc.
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.
© 2010 Pearson Education, Inc.
Particles as Waves: Electron
Diffraction
Electron diffraction
• Interference patterns of beams of light
(left) and electrons (right) compared
© 2010 Pearson Education, Inc.
Particles as Waves: Electron
Diffraction
Electron microscope uses the wave nature of electrons to
create images similar to the image of the mosquito shown
here.
© 2010 Pearson Education, Inc.
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.
© 2010 Pearson Education, Inc.
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
2, which is represented as (called h-bar).

px 

© 2010 Pearson Education, Inc.
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 .

© 2010 Pearson Education, Inc.
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  .

© 2010 Pearson Education, Inc.
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.
© 2010 Pearson Education, Inc.
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.
© 2010 Pearson Education, Inc.
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.
© 2010 Pearson Education, Inc.
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.
© 2010 Pearson Education, Inc.
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