photons
Physics 100
Chapt 21
Photoelectric effect
cathode
Vacuum
tube
anode
Photoelectric effect
Vacuum
tube
Experimental results
Electron KE
(electron Volts)
For light freq below f0,
no electrons leave the
cathode
f0
Even if the light
Is very intense
0
0.5
1.0
1.5
Experimental results
For light freq above f0,
the KE of electrons that
leave the cathode increase
with increasing freq
Electron KE
(electron Volts)
f0
0
0.5
But does not change
With light intensity
1.0
1.5
What does Maxwell’s theory say?
E
E
E
Electrons in
cathode are
accelerated by
the E-field of
the light wave
More intense light has
bigger E-fields
E
E
E
And, therefore
Larger acceleration
Electron KE should depend
on E-field strength light intensity
Electron’s motion
But that’s not what is observed
Electron KE
(electron Volts)
Below f0, no electrons jump
out of the cathode no
matter what the light’s
intensity is
0
Above f0,the KE only
depends on freq, & not
on the light’s intensity
f0
0.5
1.0
1.5
Einstein’s explanation
Light is comprised of particle-like
quanta each with energy
Equant = hf
The quanta collide with electrons &
Transfer all their energy to them
Each electron needs a minimum energy to escape
the cathode. This is called f
If Equant is less than f, the electron can’t escape
If Equant is greater than f, the electron escapes & the
quantum energy in excess of f becomes electron KE
KEelectron = hf - f
f
Light quanta  “photons”
Einstein’s light quanta
were given the name
“photons” by Arthur
Compton
Photon Energy for red light
Red light:
f = 4.0x1014 Hz
Ephoton = hf = (6.6x10-34 Js) x (4.0x1014 Hz)
= (6.6x4.0)x10-34+14 J
= 2.6 x 10-19 J
=
2.6 eV
1.6
x
= 26 x 10-20 J
1eV
1.6 x 10-19 J
=1.6 eV
Photon Energies for visible light
color:
Red
Yellow
Green
Blue
Violet
freq
4.0x1014 Hz
5.0x1014Hz
6.0x1014 Hz
6.7x1014Hz
7.5x1014 Hz
Equant = hf
2.6x10-19J
3.3x10-19J
4.0x10-19J
4.4x10-19J
5.0x10-19J
1.6 eV
2.1 eV
2.5 eV
2.8 eV
3.1 eV
Producing photoelectrons with
photons
Clears the barrier
with energy to
spare
-
-
1.6eV
KE=0.7eV
outside of
2.8eV
the metal
f=2.1eV
-
-
-
-
inside the metal
Not enough
energy to get
over the barrier
For E
Electron KE
(electron Volts)
violet
blue
yellow
red
KE
0
0.5
KE
1.0
1.5
Photons are weird particles
v=c (always)
1
1 – v2/c2
=
1
1 – 1
1
=
1 – c2/c2
=

(always)
What is the photon’s rest mass?
E=mc2
m = g m0
E
 m= 2
c
m = m =0
 m0 =
g

m0 = 0
 Rest mass = 0
Photon’s momentum
For any particle: p=mv
E
for a photon: m= 2
c
E
E
p = 2c =
c
c
& v=c
Photon energy & momentum
E = hf
E
hf
p=
=
c
c
Wavelength:
c
l =
f
h
=
l

f= 1
l
c
“particles” of light
h
p =
l
E=hf
Two body collisions
conservation
of momentum
Compton scattering
Scatter X-rays from electrons
p=h/li
-
Recoil electron &
scattered photon
conserve momentum
Compton’s expt proved the
existence of photons
& won him the
1927 Nobel Prize
(Physics)
4x10-11eV
g-rays
X-rays
Ultraviolet
Infrared
micro
waves
TV/FM
AM
radio
waves
Photon “spectrum”
4x10-7eV
4x10-3eV
4eV
4x103eV
visible light
1.6 – 3.1eV
4x106eV
Wave? Particles??
Physics 100
Chapt 22
Maxwell
E
B
James Clerk Maxwell
Light is a wave of oscillating E- and B-fields
Einstein
h
p =
l
E=hf
Light is comprised of particle-like quanta
called photons
Who’s right??
Waves explain diffraction & interference
Photons explain photoelectric effect &
Compton scattering
Impossible to explain interference
with particles
With 2 slits open
no light goes here
Block off one slit
Now light
can go here
Impossible to explain PE-effect
and Compton scattering with waves
Electron KE
(electron Volts)
yell
ow
red
0.5
violet
blue
1.0
1.5
Make an interference
pattern with low intensity light
One photon at a time goes through
the two-slit apparatus
-Light behaves like a wave when
it propagates through space
-And as a particle when it
interacts with matter
Photon photography
Photoelectric effect
Vacuum
tube