Photoelectric Effect-Classical Physics
Photoelectric Effect-What’s Going On?
“Classical” Method
Increase energy by
increasing amplitude
What if we try this ?
Vary wavelength, fixed amplitude
electrons
emitted ?
No
No
No
No
electrons
emitted ?
No
Yes, with
low KE
Yes, with
high KE
 Electrons are attracted to the (positively charged) nucleus by the
electrical force
 In metals, the outermost electrons are not tightly bound, and can
be easily “liberated” from the shackles of its atom.
 It just takes sufficient energy…
Classically, we increase the energy
of an EM wave by increasing the
intensity (e.g. brightness)
Energy  A2
No electrons were emitted until the frequency of the light
exceeded a critical frequency, at which point electrons were
emitted from the surface!
(Recall: small   large )
Enter a real Einstein
 An alternate view is that light is acting like a particle—one light particle is
needed to eject one electron
 The light particle must have sufficient energy to “free” the
electron from the atom.
But this doesn’t work ??
Einstein’s Relations:
Einstein predicted that a graph of the maximum kinetic
energy versus frequency would be a straight line, given
by the linear relation:
KE = hv - Φ
…Therefore light energy comes in multiples of hv
 Increasing the Amplitude is simply increasing the number
of light particles, and two light particles can’t add to one electron’s energy
at the same time!
 However, if the energy of these “light particle” is related to their
frequency, this would explain why higher frequency light can
knock the electrons out of their atoms, but low frequency light cannot…
More Explanations


The maximum KE increases with
increasing frequency
The effect is instantaneous since
there is a one-to-one interaction
between the photon and the
electron
Kinetic Energy Quiz
You shine light of frequency f on a metal and
notice that electrons are ejected with zero
velocity. Now you double the frequency of the
light. How much kinetic energy do the ejected
electrons now have?
A. Zero
B. hf
C. 2hf
D. hf/2
E. Can not be determined from information
given
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Photons
Quantum Mechanics
 In this “quantum-mechanical” picture, the energy of the
light particle (photon) must overcome the binding energy of the
electron to the nucleus.
 If the energy of the photon exceeds the binding energy, the
electron is emitted with a KE = Ephoton – Ebinding.
 The energy of the photon is given by E=hwhere the
constant h = 6.6x10-34 [J s] is Planck’s constant.
“Light particle”
 Quantum theory describes light as
a particle called a photon
 According to quantum theory, a
photon has an energy given by
E = h = hc/
h = 6.6x10-34 [J s] Planck’s constant,
after the scientist Max Planck.
 The energy of the light is proportional to the frequency (inversely
proportional to the wavelength) ! The higher the frequency (lower
wavelength) the higher the energy of the photon.
 10 photons have an energy equal to ten times a single photon.
Before Collision
After Collision
 Quantum theory describes experiments to astonishing precision,
whereas the classical wave description cannot.
Do photons carry momentum ?
Momentum
In physics, there’s another quantity which we hold just as
sacred as energy, and this is momentum.
DeBroglie’s proposed that the a photon not only carries energy,
but also carries momentum.
For an object with mass, momentum is given by:
But, p = mv, and photon’s have m=0, so how can it be that the
momentum is not zero??


p  mv


p  mv
p  h/
The units are: [kg] [m/s] == [kg m/s]
Unlike energy, which is a scalar, momentum is a vector. That is
it has both magnitude & direction. The direction is along the
direction of the velocity vector.
The reason it is important in physics, is, because like Energy:
TOTAL MOMENTUM IS ALWAYS CONSERVED
DeBroglie’s Relation
DeBroglie
relation
p=h/
Photons carry momentum !!!
E = hc / 
Photons also carry energy !!!
DeBroglie postulated that photons carry momentum, and their
momentum is:
p  E /c
If we substitute: E = hc/ into this equation, we get:
p  h/
Momentum carried by a photon
with wavelength 
So is light a
wave or a
particle ?
=h/p
On macroscopic scales, we can treat a large number of photons
as a wave.
Both energy & momentum are inversely proportional to the
wavelength !!!
 The highest energy photons are those which have
small wavelength (that’s why gamma rays are so dangerous)
When dealing with subatomic phenomenon, we are often dealing
with a single photon, or a few. In this case, you cannot use
the wave description of light. It doesn’t work !
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Emergent Applications…



Response is linear with light intensity
Extremely short response time
For example, night vision devices:
Photocells



Photocells are an application of the
photoelectric effect
When light of sufficiently high
frequency falls on the cell, a
current is produced
Examples

X-Rays

Electromagnetic radiation with short
wavelengths




Wavelengths less than for ultraviolet
Wavelengths are typically about 0.1 nm
X-rays have the ability to penetrate most
materials with relative ease
X-rays are produced when
high-speed electrons are
suddenly slowed down




X-rays are longer in
wavelength than visible
light.
A. True
B. False
Discovered and named by Roentgen in
1895
Production of X-rays
Opposite of Photo-Electric Effect

Streetlights, garage door openers,
elevators
Can be caused by the electron
striking a metal target
A current in the filament
causes electrons to be
emitted
These freed electrons are
accelerated toward a dense
metal target
The target is held at a higher
potential than the filament
Blackbody Radiation

An object at any temperature
emits electromagnetic radiation



Sometimes called thermal radiation
Stefan’s Law describes the total
power radiated
The spectrum of the radiation
depends on the temperature and
properties of the object
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Max Planck



1858 – 1947
Introduced a
“quantum of
action,” h
Awarded Nobel
Prize in 1918 for
discovering the
quantized nature
of energy
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