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