photons Physics 100 Chapt 21
Jan 18, 2016
photons
Physics 100
Chapt 21
Vacuumtube
Photoelectric effect
cathode
anode
Photoelectric effect
Vacuumtube
Experimental results
Electron KE (electron Volts)
f0
For light freq below f0,no electrons leave the cathode
Even if the light Is very intense
0 0.5 1.0 1.5
Experimental results
Electron KE (electron Volts)
f0
For light freq above f0,the KE of electrons that leave the cathode increases with increasing freq
But does not changeWith light intensity
0 0.5 1.0 1.5
What does Maxwell’s theory say?
E
E
E
Electrons incathode areaccelerated bythe E-field ofthe light wave
More intense light hasbigger E-fields
EE
E
And, thereforeLarger acceleration
Electron KE should depend on E-field strength light intensity
Electron’s motion
Not what is
observed
But that’s not what is observed
Electron KE (electron Volts)
f0
0 0.5 1.0 1.5
Above f0,the KE onlydepends on freq, & not on the light’s intensity
Below f0, no electrons jump out of the cathode no matter what the light’s intensity is
Einstein’s explanation
KEelectron = hf -
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
If Equant is less than , the electron can’t escape
If Equant is greater than , the electron escapes & the quantum energy in excess of becomes electron KE
Light quanta “photons”
Einstein’s light quantawere 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
= 26 x 10-20 J
= 2.6 x 10-19 J
1eV 1.6 x 10-19 J x
=
2.6 1.6
eV
=1.6 eV
Photon Energies for visible light
color: freq Equant = hf
Red 4.0x1014 Hz 2.6x10-19J 1.6 eV
Yellow 5.0x1014Hz 3.3x10-19J 2.1 eV Green 6.0x1014
Hz 4.0x10-19J 2.5 eVBlue 6.7x1014Hz 4.4x10-19J 2.8 eVViolet 7.5x1014
Hz 5.0x10-19J 3.1 eV
Producing photoelectrons with photons
-
--
-2.1eV
-Not enough
energy to getover the barrierRed photon-
Clears the barrier with energy to
spare
KE=0.7eV
Blue photon
Surfac
e
barr
ier
1.6eV
2.8eV
inside the metal
outside of
the metal
For E
Electron KE (electron Volts)
red
0 0.5 1.0 1.5
yellow
blue
violet
KEKE
Photons are weird particles
v=c (always)
11 – v2/c2
(always)
11 – c2/c2
11 – 1
What is the photon’s rest mass?
E=mc2 m= Ec2
m = m0 m0 = m
= m = 0
m0 = 0 Rest mass = 0
Photon’s momentum
For any particle: p=mv
for a photon: m=Ec2
& v = c
p = cEc2
= Ec
Photon energy & momentum
E = hf
p = Ec
= hfc
Wavelength: = cf
= h
= fc
1
“particles” of light
E=hf
hp =
Two body collisions
conservationof momentum
Compton scattering
Scatter X-rays from electrons
Recoil electron &scattered photonconserve momentum
p=h/i
p=h/f
-
Compton’s expt proved the existence of photons
& won him the 1927 Nobel Prize (Physics)
Photon “spectrum”
Ult
ra-
vio
let
Infr
a-
red
X-
rays
- rays
mic
ro
wave
srad
io
wave
sTV
/FM
AM
4x10-3eV 4x10-11eV 4eV 4x103eV 4x106eV 4x10-7eV
visible light1.6 – 3.1eV
Wave? Particles??
Physics 100
Chapt 22
Maxwell
Light is a wave of oscillating E- and B-fields
James Clerk Maxwell
E
B
Einstein
Light is comprised of particle-like quanta
called photons
E=hf
hp =
Who’s right??
Waves explain diffraction & interference
Photons explain photoelectric effect & Compton scattering
Impossible to explain interference with particles
With 2 slits openno light goes here
Block off one slit
Now lightcan go here
Impossible to explain PE-effectand Compton scattering with waves
Electron KE (electron Volts)
red
0.5 1.0 1.5
yellow
blue
violet
Make an interferencepattern 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
Vacuumtube