Chapter 27 Quantum Physics

 Understand the relationship between wavelength and intensity for blackbody radiation  Understand how Planck’s Hypothesis explained the relationship between wavelength and intensity for blackbody radiation

 Thermal Radiation  What is it?  How does it occur?

 Blackbody Radiation  What is it?  How does it work?  Graph of Intensity of BBR vs. Wavelength  The Ultraviolet Catastrophe

 1858 – 1947  He Provided the Explanation for the spectral distribution of Blackbody Radiation (1900 )  Awarded the Nobel Prize in 1918

 optical.com/bb_rad/bb_rad.htm optical.com/bb_rad/bb_rad.htm  black_body_radiation.html black_body_radiation.html

 Proposed electric oscillators called resonators  Resonators are quantized ( E n = nh f )  Resonators emit/absorb energy in discrete units called quanta (photons)  The Birth of Quantum Physics

 Understand how the photoelectric effect gives credence to the particle theory of light  Know how to use the work function to solve problems involving the photoelectric effect

 When light is incident on certain metallic surfaces, electrons are emitted  Photoelectrons  Hertz  Einstein’s (1905) explanation

 Graph of photoelectric current vs. potential difference (  V)  Current dependent on intensity  Current dependent on,  V

 Stopping Potential  KE max = e  V s  Independent of the radiation intensity  Work Function  Cutoff Wavelength  c = c/ f c = hc/ 

 Wave Theory could not explain:  Cutoff Frequency  KE max independent of intensity  KE max increases with light frequency  Electrons are emitted instantaneously  Photon Theory accounts for:   the work function  KE max = h f –   Depends only on light frequency  1 - 1interaction b/w photons and electrons  Linear relationship b/w f and KE max

 Photoelectric Cell  Street lights  Breathalyzer

 Understand the nature and production of x- rays so you can:  Calculate the shortest of x-rays that may be produced by electrons accelerated through a specified voltage

 Wilhelm Roentgen first noticed them in 1895 while studying electrical discharges  Characteristics of x-rays  Traveled at or near the speed of light  Were not deflected by electric or magnetic fields

 In 1912 Max von Laue suggested diffracting x- rays  Used atomic crystal lattice as a diffraction grating  determined the wavelength of x-rays to be about 0.1 nm

 The Production of x-rays  Electrons are accelerated through a  V of several thousand volts  Electrons collide with a metal plate  X-rays are the energy emitted when the electrons are decelerated, but why are they decelerated?  Threshold voltage

 Graph x-ray intensity vs. wavelength  Continuous broad spectrum dependent on the applied voltage. Why?  Characteristic spikes in the graph are dependent on the target material  min = hc/(e  V) Shortest wavelength radiation that can be produced

 Understand the concept of Compton scattering so you can  Describe Compton’s experiment, state the results, and how these results are explained  Account for the increase of photon wavelength, and explain the significance of the Compton wavelength

 Compton’s Experiment  X-ray beam of a specified wavelength, 0, directed at a block of graphite  Result was the scattered x-rays had a longer wavelength,,  Amount of energy reduction depended on the angle at which the x-rays were scattered  The change in wavelength, , is the Compton Shift.

 Compton’s Explanation  Photon  particle collision similar to billiard ball collisions. Which means what?   =  –  0 = h/(m e c) *(1 – cos  )  Compton Wavelength, h /(m e c), is very small compared to visible light.

 Understand the concept of DeBroglie wavelength so they can:  Calculate the wavelength of a particle as a function of its momentum  Describe the Davison-Germer experiment, and explain how it provides evidence for the wave nature of electrons

 Energy of a photon is converted completely into mass, pair production  Electron and positron are created from a photon  Energy, momentum, & charge are conserved

 Minimum energy required to produce a positron  h f min = 2m e c 2 ( E = mc 2 )  Pair production cannot occur in a vaccum, but can only occur in the presence of a massive particle (nucleus)

 Pair Annihilation: electron-positron pair produce two photons  Momentum has to be conserved

 Double slit experiment shooting particles at a double slit Double slit experiment shooting particles at a double slit

 Photoelectric Effect  Compton Effect  X-Rays  Pair Production & Annihilation