Download L7 Photoelectric Effect

Particle Properties of Waves
Chapter 2
Photoelectric Effect
!
Two metal plates in a vacuum
!
Shine light on one of them
!
Measure current between the plates
Photoelectric Effect
!
Two metal plates in a vacuum
!
Shine light on one of them
!
Measure current between the plates
!
Can also apply voltage V across the
plates and see the effect on the
current
!
Adjust: (1) frequency & (2) intensity
of light and (3) applied voltage.
–
+
Photoelectric Effect: Classical Prediction
Electrons gain KE, and “spill out”
!
How does changing the applied
voltage affect the current?
0 Voltage
B
0 Voltage
–
+
Current
!
Current
Light heats the metal
Current
A
!
C
0 Voltage
D
0 Voltage
Photoelectric Effect: Classical Prediction
!
Light heats the metal
!
Electrons gain KE, and “spill out”
!
How does changing the applied
voltage affect the current?
–
+
C
Diode Curve
0 Voltage
Photoelectric Effect: Classical Prediction
!
Why is the current 0 when the
voltage is reversed?
!
Electrons are repelled by cathode
and attracted to anode
+
–
C
0 Voltage
Photoelectric Effect: Classical Prediction
!
Why is the current-voltage curve flat
for positive voltage?
!
Increasing (+) voltage, accelerates
electrons to higher speed
!
But, # electrons/sec is set by rate at
which they are produced, not how
fast they travel to other plate.
!
–
+
C
0 Voltage
Thus, the current is constant.
Photoelectric Effect: Classical Prediction
!
Classical Prediction:
!
Diode current-voltage curve
!
If leave light on longer, plate heat up
more, more electrons will be ejected,
and higher current
!
!
If increase intensity, plate heats up
more giving higher current
Frequency (color) has no effect.
–
C
0 Voltage
+
Photoelectric Effect: Einstein’s Interpretation
!
Light comes in small quanta (photons)
!
Energy of a photon is E = h! where h is Plank’s constant
!
When light strikes the metal, there is a chance that a given
photon’s energy will be absorbed by an electron
!
In order for an electron to escape the metal, it must acquire a
minimum energy !, which is called the work function of the metal
!
When electrons escape, their kinetic energy is determined by the
energy of the photon and work needed to escape the metal:
h! = " + KEmax
Photoelectric Effect: Einstein’s Interpretation
h! = " + KEmax
h!
E
KEmax
!
Potential Energy of Metal
KEmax
Sodium
Calcium
Cesium
!
Work Functions of Metals
Sodium
2.3 eV
Potassium
2.3 eV
Calcium
2.9 eV
Uranium
3.6 eV
Copper
4.7 eV
Carbon
4.8 eV
Gold
5.2 eV
Platinum
6.4 eV
I
I
Which graph represents current-voltage curve
for low and high intensity light?
B
A
0 Batt. V
D
0 Batt. V
I
C
I
I
0 Batt. V
0 Batt. V
F
0 Batt. V
13
Which graph represents current-voltage curve
for low and high intensity light?
HIGH intensity
LOW intensity
I
Fewer electrons pop off metal
Current decreases.
Current proportional to light intensity.
ans. B
0
14
Battery Voltage
What happens to the initial KE of the electrons as the
frequency of light changes? (Light intensity is constant)
Predict shape
of the graph
Initial KE
e s
0
I
Frequency of light
16
C
0
Initial KE
Frequency
0
Initial KE
0
B
D
Frequency
Frequency
Initial KE
A
Initial KE
What happens to the initial KE of the electrons as the
frequency of light changes? (Light intensity is constant)
0
Frequency
17
E. something different
a. fewer electrons kicked out
b. same # of electrons
c. more electrons kicked out
d. not enough information
Electron potential
energy
You initially have blue light shining on metal. If you change the
frequency to violet light (at same # of photons per second),
what happens to the number of electrons coming out?
Ephot
Ephot
work function !
Inside
metal
You initially have blue light shining on metal. If you change the
frequency to violet light (at same # of photons per second),
what happens to the number of electrons coming out?
elect. potential
energy
Electrons over large range of energy have equal
chance of absorbing photons.
Ephot
work function !
metal
c. more electrons come out with violet
absorb blue light and have enough energy to leave
absorb blue light, but don t come out
so the more energy the light has, the more electrons that come
out, until so much energy that every electron comes out.
28
(violet and ultraviolet would not be very different in this case)
You initially have blue light shining on metal. If you change the
frequency to violet light (at same # of photons per second),
what happens to the number of electrons coming out?
Blue Light
E
h!
h!
Violet Light
!
A photon at 300 nm will kick out an electron with an amount of
kinetic energy, KE300. If the wavelength is halved to 150 nm and
the photon hits an electron in the metal with same energy as the
previous electron, the energy of the electron coming out is:
300 nm
a. < ! KE300.
b.
! KE300
c.
= KE300
d.
2 KE300
e. > 2 KE300
150 nm
A photon at 300 nm will kick out an electron with an amount of
kinetic energy, KE300. If the wavelength is halved to 150 nm and
the photon hits an electron in the metal with same energy as the
previous electron, the energy of the electron coming out is:
h! = " + KEmax
2h!
E
h!
KE150
KE300
!
!
e.
> 2 KE300
X-Rays
Wilhelm Roentgen 1895
X-Ray Tube
X-Ray Tube
Mr. Roentgen X-Rays Mrs. Roentgen’s Hand
X-Ray Production
Path of electron
Bremsstrahlung Radiation
X-rays
Normally, deflections off
atoms are gradual, and
electron KE is transferred to
KE of medium (heating it)
Large accelerations cause the
electron to radiate
Bremsstrahlung radiation
(X-rays in this case)
X-Ray Production
Einstein observed that X-ray production is the opposite of the
photoelectric effect:
photoelectric effect:
light
electron
metal
X-ray production
electron
X-rays
metal
X-Ray Spectra
X-ray spectra can have continuous
and emission line features.
The minimum wavelength of the
continuum is given by the DuaneHunt law:
!min =
1240 nm
V
where V = accelerating voltage
The minimum wavelength occurs
when all of the electron’s energy is
converted to a single photon
Derivation of Duane-Hunt Law
Since the energy of electrons in an
X-ray tube is >> work function !
h! max = " + KEmax # KEmax
hc
= KEmax
!min
!min =
hc 1240 nm
=
eV
V
X-Rays as EM Radiation
Initially, X-rays were mysterious:
!
They were not bent by magnetic fields (like cathode rays)
!
They did not show diffraction or interference effects like
light (why?)
!
Bragg showed that X-rays actually are waves by
diffracting them through planes of atoms in crystals
Compton Effect
Initially, X-rays were mysterious:
!
They were not bent by magnetic fields (like cathode rays)
!
They did not show diffraction or interference effects like
light (why?)
!
Bragg showed that X-rays actually are waves by
diffracting them through planes of atoms in crystals
Compton Effect
Photon scattering off an electron acts like two particles in an
elastic collision
Energy Conservation
scattered photon
! = h# "
photon
h! " h! # = $KEelectron
!
! = h"
"
!=
electron
( pc )2 + ( mc 2 )
2
Compton Effect
Photon scattering off an electron acts like two particles in an
elastic collision
Momentum Conservation
scattered photon
p=
photon
p=
h" !
c
!
h!
c
"
photon
p=
h!
c
p
electron
electron
Compton Effect
Momentum Conservation
photon
p=
h!
c
electron
h" !
sin #
c
!
"
p sin !
p
h" !
cos#
c
p cos !
h" !
sin #
c
!
"
p sin !
p
h" !
cos#
c
p cos !