The photoelectric effect Contents: Einstein’s proposed experiment Solving photoelectric problems Example 1 | Example 2Example 1Example 2 Whiteboard Photon vs wave theory

Waves Photons Color Brightness Energy per photon changes E = hf X-Rays, UV, Gamma Wavelength Changes Amplitude Changes # of photons changes many = bright few = dim CCD Devices, High speed film big = red small = blue small = dim big = bright Light

Einstein Proposes Photon theory Experiment: Light ejects electrons Ammeter detects +- V Reverse the voltage Some Potential V stops all electrons This is called the “Stopping potential” (Pretty tough, huh?) TOC

Remember V = W/q, so W = Vq Definition of electron volt E kin turns to potential energy If 5.12 V is the stopping potential, then E kin = 5.12 eV - + Electric Force Consider an ejected electron hurtling toward the negative plate:

TOC Einstein’s Photon theory predicts: Photon energy = Work function + Kinetic energy of electron hf =  + E kmax hf = hf o + eV s  - Work function (Depends on material) f o - Lowest frequency that ejects e - Electron charge V s - The uh stopping potential +- V

Ag (silver)4.26 Au (gold)5.1 Cs (cesium)2.14 Cu (copper)4.65 Li (lithium)2.9 Pb (lead)4.25 Sn (tin)4.42 Chromium4.6 Nickel4.6 Metal Work Function

TOC Photon energy = Work function + Kinetic energy of electron hf =  + E kmax hf = hf o + eV s  - Work function f o - Lowest frequency that ejects e - Electron charge V s - The uh stopping potential e = x C V = W/q E = hf = hc/ Example 1: A certain metal has a work function of 3.25 eV. When light of an unknown wavelength strikes it, the electrons have a stopping potential of 7.35 V. What is the wavelength of the light?

TOC Photon energy = Work function + Kinetic energy of electron hf =  + E kmax hf = hf o + eV s  - Work function f o - Lowest frequency that ejects e - Electron charge V s - The uh stopping potential e = x C V = W/q E = hf = hc/ Example 2: 70.9 nm light strikes a metal with a work function of 5.10 eV. What is the maximum kinetic energy of the ejected photons in eV? What is the stopping potential?

Whiteboards: Photoelectric effect 11 | 2 | 3 | 4234 TOC

6.00 eV W Photon energy = Work function + Kinetic energy of electron Photon energy = 2.15 eV eV = 6.00 eV Photons of a certain energy strike a metal with a work function of 2.15 eV. The ejected electrons have a kinetic energy of 3.85 eV. (A stopping potential of 3.85 V) What is the energy of the incoming photons in eV?

147 nm W E = hf = hc/, Photon energy = Work function + Kinetic energy of electron Photon energy = 3.46 eV eV = 8.46 eV E = (8.46 eV)(1.602 x J/eV) = x J E = hf = hc/, = hc/E = (6.626 x Js)(3.00 x 10 8 m/s)/( x J) = x m = 147 nm Another metal has a work function of 3.46 eV. What is the wavelength of light that ejects electrons with a stopping potential of 5.00 V? (2)

6.67 V W E = hf = hc/, 1 eV = x J Photon energy = Work function + Kinetic energy of electron E = hf = hc/ = (6.626 x Js)(3.00 x 10 8 m/s)/(112 x m) E = x J E = ( x J)/(1.602 x J/eV) = eV Photon energy = Work function + Kinetic energy of electron eV = 4.41 eV + eV s eV eV = eV = eV s V s = 6.67 V 112 nm light strikes a metal with a work function of 4.41 eV. What is the stopping potential of the ejected electrons? (2)

3.70 eV W E = hf = hc/, 1 eV = x J Photon energy = Work function + Kinetic energy of electron E = hf = hc/ = (6.626 x Js)(3.00 x 10 8 m/s)/(256 x m) E = x J E = ( x J)/(1.602 x J/eV) = eV Photon energy = Work function + Kinetic energy of electron eV = Work function eV eV eV = 3.70 eV 256 nm light strikes a metal and the ejected electrons have a stopping potential of 1.15 V. What is the work function of the metal in eV? (2)

TOC Einstein’s Photon theory predicts: Photon energy = work function + Kinetic energy of electron hf =  + E kmax E kmax = hf -  Photon Theory predicts: E kmax rises with frequency Intensity of light ejects more Wave Theory predicts: E kmax rises with Amplitude (Intensity) Frequency should not matter Survey says Millikan does the experiment 1915 conclusion 1930 conclusion