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
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
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
Compton scattering
Scatter X-rays from electrons
Recoil electron &scattered photonconserve momentum
p=h/i
p=h/f
-
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
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