photons Physics 100 Chapt 21 Photoelectric effect cathode

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 f 0, no electrons leave the cathode f 0 Even if the light Is very intense 0 0. 5 1. 0 1. 5

Experimental results For light freq above f 0, the KE of electrons that leave the cathode increase with increasing freq Electron KE (electron Volts) f 0 0 0. 5 But does not change With light intensity 1. 0 1. 5

What does Maxwell’s theory say? 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 e s b d e rv t o N o s i t a wh

But that’s not what is observed Electron KE (electron Volts) Below f 0, no electrons jump out of the cathode no matter what the light’s intensity is 0 Above f 0, the KE only depends on freq, & not on the light’s intensity f 0 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. 0 x 1014 Hz Ephoton = hf = (6. 6 x 10 -34 Js) x (4. 0 x 1014 Hz) = (6. 6 x 4. 0)x 10 -34+14 J = 2. 6 x 10 -19 J = 2. 6 e. V 1. 6 x = 26 x 10 -20 J 1 e. V 1. 6 x 10 -19 J =1. 6 e. V

Photon Energies for visible light color: Red Yellow Green Blue Violet freq 4. 0 x 1014 Hz 5. 0 x 1014 Hz 6. 0 x 1014 Hz 6. 7 x 1014 Hz 7. 5 x 1014 Hz Equant = hf 2. 6 x 10 -19 J 3. 3 x 10 -19 J 4. 0 x 10 -19 J 4. 4 x 10 -19 J 5. 0 x 10 -19 J 1. 6 e. V 2. 1 e. V 2. 5 e. V 2. 8 e. V 3. 1 e. V

Producing photoelectrons with photons Clears the barrier with energy to spare rr ie r KE=0. 7 e. V 2. 8 e. V Su rf a ce ba outside of the metal f=2. 1 e. V - - - 1. 6 e. V Not enough energy to get over thehbarrier oton p d e R - inside the metal oto h p Blue n

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 – v 2/c 2 = 1 1 – 1 1 = 1 – c 2/c 2 = (always)

What is the photon’s rest mass? E=mc 2 m = g m 0 E m= 2 c m m = =0 m 0 = g m 0 = 0 Rest mass = 0

Photon’s momentum For any particle: p=mv E for a photon: m= 2 c E E p = 2 c = 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 - p= h/ Recoil electron & scattered photon conserve momentum l f

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

4 x 10 -11 e. V g-rays X-rays Ultraviolet Infrared micro waves TV/FM AM radio waves Photon “spectrum” 4 x 10 -7 e. V 4 x 10 -3 e. V 4 x 103 e. V visible light 1. 6 – 3. 1 e. V 4 x 106 e. V

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 violet blue red 0. 5 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