Photoelectric Effect – Quantum Physics
© 2010 Pearson Education, Inc.
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Learning Objectives:
 Recognize terms related to the photoelectric
effect experiment
 Analyze data collected during the laboratory
experiment
 Recognize the need for new ways to think about
light and matter at the atomic level
 Apply the photon model of light to explain the
photoelectric effect
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The Photoelectric Effect: PhET Lab
 Current is directly
proportional to light
intensity.
Electrons are emitted only if the
light frequency exceeds a threshold
frequency fo.
 Current appears
without delay when the
light is applied.
Value of the threshold frequency fo
depends on the type of metal from
which the cathode is made.
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Stopping Potential
 If the potential difference Δv is
positive (anode + with respect to
cathode), current changes very little
as Δv is increased. By reversing the
battery, Δv is made − (anode − with
respect to cathode), the current
decreases until at some voltage Δv =
−vstop the current reaches zero.
The value of vstop is called the
stopping potential.
NOTE: The − sign means the
potential difference that stops
the electrons.
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 Value of vstop is the same for both
weak and intense light. A more
intense light causes a greater current,
but both cease when Δv = −vstop
The Effect of Voltage Between Anode and
Cathode
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Classical Physics Explanation
 Early investigations showed a heated electrode emitted
electrons spontaneously, so it was natural to suggest that the
light falling on the cathode simply heated it, causing electrons
to be emitted.
 Following this reasoning, if a weak intensity of light at a
particular frequency (which just happened to be at the fo) can
generate a current, then certainly a strong intensity at a
frequency slightly lower, should also be able to do so – it
will heat the metal even more. A slight change in
frequency should not matter.
 However, as you discovered in your experiment, a
specific frequency fo was required to eject electrons from
the various metals you tested.
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A New Physical Theory Is Needed to Explain The Data
 1905 – Einstein published three papers on three different
topics.
- Theory of relativity
- Statistical mechanics explaining Brownian motion
- Nature of light
 Einstein’s third paper offered a simple but bold idea that
explained the data from the photoelectric experiment.
Electromagnetic radiation is quantized! Light is not a
continuous wave but, instead, arrives as small packets of
energy (light quanta). Each quantum of light (photon) has
energy directly proportional to its frequency.
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Photons: Light Quanta
If the photon has enough
energy (greater than the work
function) an electron is emitted.
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Recall: The Photon Model of EM Waves
Basic Postulates:
 EM waves consist of mass-less units called
photons.
 Each photon has energy
Ephoton  hf
where f is frequency of the wave and h is a
universal constant called Planck’s constant.
h  6.63 10-34 J  s = 4.14 x 10-15 eV ∙ s
 Superposition of a large number of photons
has the characteristics of a continuous
electromagnetic wave.
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Einstein’s Postulates – Underpinnings of the Photon
Model
 Light of frequency f consists of discrete quanta, each of
energy E = hf . Each photon travels at the speed of
light c.
 Light quanta are emitted or absorbed on an all-ornothing basis. A substance can emit 1 or 2 or 3 quanta,
but not 1.5. Similarly, an electron in a metal cannot
absorb half a quantum but, instead, only an integer
number.
 A light quantum, when absorbed by a metal, delivers its
entire energy to one electron.
These three postulates explain light quanta and their
interaction with matter.
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Matter Waves
An electron beam
passing through a
double slit produces
an interference pattern
similar to that for light.
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Energy Levels and Quantum Jumps
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Summary: 28.1 - 28.3
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Summary
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Photon Interactions and Properties
 Photoelectric Effect
 Compton Scattering
 Pair Production
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Photoelectric Effect
Ejected
Electron
Incoming
Photon
Photoelectric Effect: Predominates with incident photons of
low energy; complete energy transfer of the incoming photon
to the ejected electron
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Compton Scattering
Incoming
Photon, Eo
Ejected
Electron, v
q
f
Scattered
Photon, E
Compton Scattering: Predominates with incident photons of medium
energy; partial energy transfer of the incoming photon to the ejected
electron and the scattered photon.
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Pair Production
Incoming
Photon
+
-
Positron
Emitted
Annihilation Electron
Photons
Pair Production: Predominates with incident photons of high
energy (at least 1.02 MeV); Positron and electron formed, then
two 0.51 MeV annihilation photons are emitted
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Atom Excitation
A photon may knock an atomic electron e- to a
higher energy state when absorbed by the atom.
The atom is said to be in an excited state.
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