photons Physics 100 Chapt 21

Vacuum tube Photoelectric effect cathode anode

Photoelectric effect Vacuum tube

Experimental results Electron KE ( electron Volts) f0f0 For light freq below f 0, no electrons leave the cathode Even if the light Is very intense

Experimental results Electron KE ( electron Volts) f0f0 For light freq above f 0, the KE of electrons that leave the cathode increases with increasing freq But does not change With light intensity

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 Not what is observed

But that’s not what is observed Electron KE ( electron Volts) f0f Above f 0,the KE only depends on freq, & not on the light’s intensity Below f 0, no electrons jump out of the cathode no matter what the light’s intensity is

Einstein’s explanation  KE electron = hf -  Light is comprised of particle-like quanta each with energy E quant = 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 E quant is less than , the electron can’t escape If E quant is greater than , the electron escapes & the quantum energy in excess of  becomes electron KE

Light quanta  “photons” Einstein’s light quanta were given the name “photons” by Arthur Compton

Photon Energy for red light Red light: f = 4.0x10 14 Hz E photon = hf = (6.6x Js) x (4.0x10 14 Hz) = (6.6x4.0)x J = 26 x J = 2.6 x J 1eV 1.6 x J x = eV =1.6 eV

Photon Energies for visible light color: freq E quant = hf Red 4.0x10 14 Hz 2.6x J 1.6 eV Yellow 5.0x10 14 Hz 3.3x J 2.1 eV Green 6.0x10 14 Hz 4.0x J 2.5 eV Blue 6.7x10 14 Hz 4.4x J 2.8 eV Violet 7.5x10 14 Hz 5.0x J 3.1 eV

Producing photoelectrons with photons   2.1eV - Not enough energy to get over the barrier Red photon - Clears the barrier with energy to spare KE=0.7eV Blue photon Surface barrier 1.6eV 2.8eV inside the metal outside of the metal

For E Electron KE ( electron Volts) red yellow blue violet KE

Photons are weird particles v=c (always) 1  1 – v 2 /c 2   (always) 1  1 – c 2 /c 2  1  1 – 1 

What is the photon’s rest mass? E=mc 2  m= Ec2Ec2 m =  m 0  m 0 = mm = mm = 0 = 0 m 0 = 0  Rest mass = 0

Photon’s momentum For any particle: p=mv for a photon: m= Ec2Ec2 & v = c p = c Ec2Ec2 = EcEc

Photon energy & momentum E = hf p = EcEc = hf c Wavelength: = cfcf = h  = fcfc 1

“particles” of light E=hf h p =

Two body collisions conservation of momentum

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

Compton’s expt proved the existence of photons & won him the 1927 Nobel Prize (Physics)

Photon “spectrum” Ultra- violet Infra- red X-rays  -rays micro waves radio waves TV/FM AM 4x10 -3 eV4x eV4eV 4x10 3 eV 4x10 6 eV 4x10 -7 eV visible light 1.6 – 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 h p =

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) red yell ow blue violet

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