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The Particle of Light
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A particle model of light is necessary to describe
phenomena observed in modern physics, for example,
the interaction between light and atoms.
PHYS140
Light as a Particle
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The Photoelectric Effect
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Many physicists’ work contributed to the discovery
of the photoelectric effect
What is it?
• The ability of light to dislodge electrons from a
metallic surface
• The electrons can be detected and the resulting
signals amplified
• Lots of applications in visual imaging
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Light as a Particle
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Questions
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How many electrons are ejected in a given time?
How does this number depend of wavelength or
intensity?
How energetic are the ejected electrons?
Upon what does the electron energy depend?
Are electrons ejected instantly or is there a time
delay?
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Light as a Particle
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Photoelectric Experiments
Experiment 2
Experiment 1
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Cathode – electrons are ejected
Anode – electrons are collected
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Light as a Particle
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Photoelectric Experiments - con’t
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a) Electrons freely flow from the anode back to the
cathode and they are counted along the way
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Can determine how # of e- depends on
wavelength and intensity; time light must shine
on cathode for electrons to flow
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Photoelectric Experiments - con’t
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b) Ejected electrons have to overcome the electric
field to get to the anode
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Light as a Particle
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Photoelectric Experiments - con’t
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b) Ejected electrons have to overcome the electric
field to get to the anode
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Can determine energy of ejected electron
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If the potential difference between the plates,
ΔΦ = 2.0 V, the difference between the
electron’s electrostatic potential energy at the
anode and its potential energy at the cathode is
q  1.6 1019 C2.0V   3.2 1019 J
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The electron can make it to the anode only if it
has an initial kinetic energy greater than this
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Light as a Particle
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Wave Model Predictions
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The rate at which electrons are ejected from a metal is
proportional to the intensity of the incident light.
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Lower intensity light rays should have a delay before
electrons are ejected
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The rate may depend on frequency (wavelength) of light
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The maximum kinetic energy of the electrons is likely to
increase with increasing intensity
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Experiments Provide the
Following Results
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At high intensities and fixed frequencies, the #
of ejected electrons is proportional to intensity
Electrons are ejected instantly, regardless of
intensity level
For constant intensity, the # of electrons
decreases with increasing frequency
If the frequency is below a certain level, no
electrons are ejected, regardless of intensity
level
Above the cutoff frequency, the electrons’
maximum kinetic energy is proportional to the
frequency of light
PHYS140
Light as a Particle
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✔
✔
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✔
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Maximum Energy depends on Frequency
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Above the cutoff frequency, the electrons’
maximum kinetic energy is proportional to the
frequency of light
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Light as a Particle
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Einstein’s Prediction – light is a particle
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Light consists of particles, each carrying a certain
amount of energy
E  hf 
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hc

Where E is the energy, f is the frequency, and h is
Planck’s constant
h  6.63 1034 J  s  4.15 1015 eV  s
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We typically express colors of light in wavelengths


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f 
c

hc 1240eV  nm
Light as a Particle
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Einstein’s Prediction - con’t
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Einstein also predicted that each electron ejected
from the metal was a result of a collision with a
single photon
K
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hc

W
Where K is the kinetic energy of the electron and W
is the work function for the metal
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
PHYS140
The work function is the energy required to liberate the
electron from the metal
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Einstein’s Prediction - con’t
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Einstein’s model explains the experimental results so
neatly, why was there resistance in the science
community?
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This model is completely inconsistent with the wave nature
of light.
Neither model, wave or particle, adequately
explains light by itself
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Practice
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Interactive activity – photoelectric effect for
different metals
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http://phet.colorado.edu/simulations/sims.php?sim
=Photoelectric_Effect
Group Problems
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PHYS140
Q3B.5, Q3S.4
Light as a Particle
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