Physics 361 Principles of Modern Physics Lecture 2

Photoelectric Effect When light strikes a metal, electrons (or “photoelectrons”) can be dislodged from metal. Energetic diagram e-e- e-e- Vacuum Level Weakest-Bound Electrons EFEF   work function 

Work Function e-e- Vacuum Level Weakest-Bound Electrons EFEF   work function  Work functions of common metals

Photoelectric Effect (Experiment) When light strikes E, photoelectrons are emitted Electrons collected at C and passing through the ammeter create a current in the circuit C is maintained at a positive potential by the power supply

What Property of Light Determines Its Ability to Dislodge Electrons? Size of wave (“Amplitude”)? e-e-

What Property of Light Determines Its Ability to Dislodge Electrons? Size of wave (“Amplitude”)? e-e- NO

It’s the Frequency or Wavelength that matters! When light strikes E, photoelectrons are emitted Electrons collected at C and passing through the ammeter create a current in the circuit C is maintained at a positive potential by the power supply No electrons are emitted if the incident light frequency is below some cutoff frequency that is characteristic of the material being illuminated

Einstein’s Explanation The photon’s energy would be E = hƒ, with h a universal constant. Each photon can give all its energy to an electron in the metal The maximum kinetic energy of the liberated photoelectron is KE max = hƒ – Φ Φ is the work function of the metal

Verification of Einstein’s Theory Experimental observations of a linear relationship between KE and frequency confirm Einstein’s theory The x-intercept is the cutoff frequency

Cutoff Wavelength The cutoff wavelength is related to the work function Wavelengths greater than C incident on a material with a work function  don’t result in the emission of photoelectrons Can write cutoff relation in terms of f or through