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AN INTRODUCTION TO…
MODERN PHYSICS
Let There be Light…
• In 1873, James Clerk Maxwell summarized in 4
equations everything that was known about EM
and EM waves
They state (basically):
1. Electric field lines start on a positive
charge and end on a negative charge.
2. Magnetic field line form closed loops
and have no beginning or end
Maxwell’s Thoughts cont…
3. An electric field must exist around a
changing magnetic field.
4. A changing electric field generates a
magnetic field.
These equations were thought to be the
“be all and end all” of Physics. They fully
explained everything known about
electromagnetic waves. They were used
to predict the speed of these waves.
Catastrophe!
• The first problem for Maxwell’s eq’ns
started with incandescence.
I’ve got a bad
feeling about
this…
James
Maxwell
Gustav Kirchoff
Catastrophe!
• Incandescence: heating matter until
it emits electromagnetic radiation.
• Kirchoff (1824-1887) thought of a
“blackbody radiator”
According to Maxwell’s eq’ns:
PαT4 and fmax α T
Catastrophe!
• This relationship worked- sort of….
• It worked until the frequency of light
emitted was higher than visible light.
• This was what was expected:
Catastrophe!
Catastrophe!
• This is what occurred:
p
Catastrophe!
• This was called the UV catastrophe –
the top minds of the day couldn’t
figure out why this occurred.
Enter Max Planck- one of Kirchoff’s
grad students
Planck to the rescue…
• Planck came up with a formula to get
the curve we saw earlier. It was an
empirical model.
The theory behind this model was
that ENERGY WAS QUANTIZED!
Energy was made up of discrete energy
amounts
The Beginning of Quantum
Physics
• Planck found that the vibrations of
the atoms in the radiator had certain
energy levels.
• He found that the energy levels were
a constant multiplied by the
frequency: E = hf
Finally a constant
that’s not a k! That’s
OK!
The Beginning of Quantum
Physics
• The energy of the oscillations were
found to be: 0hf, 1hf, 2hf, 3hf, …
• So, E = nhf, where n = 0, 1,2,3,…
• ‘h’ is called Planck’s constant and is
h = 6.626 x 10 ^-34 Js
Planck’s model was purely
mathematical and worked
only if energy was quantized.
The theory behind this was
still a mystery.
Shedding light on Problem #2
• Physicists’ understanding of EM was
challenged with another experiment:
The Photoelectric Effect
In the experiment, light was incident
on a metal plate which was attached
to a wire. The wire was connected to
a galvanometer (a sensitive
ammeter).
The Photoelectric Effect
A simulation of the Photoelectric
effect
The Photoelectric Effect
When certain light shines on the
metal, current passes through the
ammeter. What is happening?
It must be that electrons are being
ejecting from the metal. But why?
The Photoelectric Effect
Philipp Lenard also figured out that if
you switch the battery poles, you can
stop the electrons from hitting the
collector plate. But again…why?
The Photoelectric Effect
To solve these questions, physicists
called on probably the greatest
physicist ever…
The Photoelectric Effect
Einstein developed the theory behind
this and won the Nobel Prize for it.
To fully understand the significance of Einstein’s
theory we must look at what physicists expected to
happen and then we really did occur during the
experiment.
The Photoelectric Effect
Experiment 1:Change the colour of light
Use applet and red to purple with cobalt to show that the effect does not occur with all light. (I = 50%), V = 0V
What we saw was that when CERTAIN light
hitting a metal, electrons from the metal were
ejected. From now on these electrons will be
called PHOTOELECTRONS indicating they
were emitted through the photoelectric effect.
Not all light caused the photoelectric effect to occur!
The Photoelectric Effect
Experiment 2:Change the intensity of light
Use applet and red increase the intensity to 100%. use cobalt to show that the effect does not occur
Use purple and increase the intensity to see more current
Return it to red and leave running
What we saw was that the intensity of the light
DOES NOT affect whether or not the electrons
are ejected.
If the photoelectric effect occurs, increasing the
intensity will increase the current.
The Photoelectric Effect
Classical physics predicts that changing the
intensity will increase the energy of the wave and
thus the electrons should be emitted with higher
intensity.
Remember: for a wave I = P/A = E/t/A so I α E.
More intense waves carry more energy.
But this doesn’t happen!
The Photoelectric Effect
Since light is a wave
resonance must be the factor
behind the emitted electrons. Light strikes an electron at the
electron vibrates with the frequency of the light ray. As more and
more light rays hit this electron, it will gain energy. (Think pushing
someone on a swing.) This is possible since resonance can do
this…
Example 2
The Photoelectric Effect
If resonance is the cause then maybe the red light
needs more TIME to cause the electrons to be
ejected. Let’s look back at the simulation.
The Photoelectric Effect
Still no current. The effect either
occurs with that light or it never will.
This is NOT what classical physics
predicts.
Experiment 3: Introducing a potential
Use a neg voltage to show that the current drops but not to zero. Some of the electrons are
stopped but not all. Increase the voltage to show the stopping potential
Experiment 3
We see that adding a negative voltage drops the
current in the circuit. Why?
Some electrons are not able to pass through the
potential we’ve created.
Board
Experiment 3
We see that adding a negative voltage drops the
current in the circuit. Why?
Some electrons are not able to pass through the
potential we’ve created.
Potential
Experiment 3
What can we conclude about the marbles if they
travelled different distances up the ramp?
They must have different amount of kinetic energy. So too
must the photoelectrons since only some of them pass
through the potential barrier (voltage). Show PhEt sim
However, if light is a wave then the intensity is related to the
energy it has. So if we change the intensity, we can change the
stopping potential. But this does not happen!
What we see now shows us that
1) that the photoelectric effect occurring is independent
of the light’s intensity but is dependant on the light’s
frequency.
2) the photoelectrons are emitted without delay
3) the photoelectrons have different energies and this
energy is independent on the intensity of the light.
Classical Physics is…
The Photoelectric Effect
Einstein (who never worked in the lab!) studied
these results and came up with the idea that light
is a PARTICLE, not a wave.
He took Planck’s idea of energy being quantized
and applied it to light. He called a packet of light
energy a PHOTON. Each photon of light has a
certain energy pertaining to its colour (its
frequency).
The Photoelectric Effect
If the photon has the right energy, it can knock the
electron off the metal.
Einstein also recalled Lenard’s contribution:
reversing the polarity of the battery enables us to
control and stop the current.
Einstein realized that what was happening was
that these photoelectrons had different energies.
That’s why when the polarity was reversed, the
current decreases – some of the electrons are
“sucked back”.
The Photoelectric Effect
The Photoelectric Effect
When all the electrons are sucked back, the
voltage is called the stopping potential. It has
enough potential to “use up” the kinetic energy of
the most energetic electron.
Remember:
EQ  V * q
ping Potential
The Photoelectric Effect
So the potential
energy the Frisbee
has here…
This height represents the “stopping
potential” of the Frisbee
equals the kinetic energy
it had here!
The Photoelectric Effect
Experiment 4: Finding the stopping potential:
λ = 494nm
I = 100%
V = -0.20V
Unclick Show electrons with max energy only.
The Photoelectric Effect
By measuring the voltage require to stop the
photoelectrons from hitting the collector plate, we
are calculating ….
We know now the MAXIMUM kinetic energy of a
photoelectron for the experiment. Why do
photoelectrons have different energies anyways?
The Work Function
Let’s take a close look at the surface of a metal:
The first electron used some of the energy to move to the top of
the surface. Then it used some to be ejected off the surface. Its
kinetic energy was the left over energy. The second electron
was already at the top of the surface so it just lost the energy
required to be ejected: the work function! That photoelectron will
have the max. kinetic energy.
The Results: Graphically
Work
Function
Threshold
frequency
The Equation of the Line
As we know the equation of a line is y =mx+b. This
graph is no different:
KE(max) = hf - W
A few things to remember:
The energy of a photon = hf
The threshold frequency, f0,
and the work function are
related: W=hf0
The KE(max) is measured in eV.
1eV = 1.6 x 10^-19J
The KE(max) is calculated by
using the stopping potential (V*q)
Examples
Calculate the energy of the green photon and the
velocity of the photoelectron (blue incident light)
Photoelectric Effect
Problems
1.Calculate the energy of a photon of blue light with a frequency of
6.67 x 1014 Hz. (State in eV) [2.76eV]
2.Calculate the energy of a photon of red light with a wavelength of
630 nm. [1.97eV]
3. Light of wavelength 600nm is directed at a metallic surface with a
work function of 1.60 eV. Calculate
a. the maximum kinetic energy of the photoelectron. [Ek = 7.6 x
10-20]
b. the maximum speed of the photoelectron. [v = 4.1 x 105m/s]
c. the stopping potential of the metal. [v0 = 0.48 v]
4. Barium has a work function of 2.48 eV. What is the maximum
kinetic energy of the ejected electron if the metal is illuminated by
light of wavelength 450 nm? [0.28 eV]
5. When a 350nm light ray falls on a metal, the maximum kinetic
energy of the photoelectron is 1.20eV. What is the work function
of the metal? [2.3 eV]
Blackbody
a small hole in the side of a large box is an
excellent absorber, since any radiation
that goes through the hole bounces
around inside, a lot getting absorbed on
each bounce, and has little chance of ever
getting out again. So, we can do this in
reverse: have an oven with a tiny hole in
the side, and presumably the radiation
coming out the hole is as good a
representation of a perfect emitter as
we’re going to find.