The Particle of Light

1. The Particle of Light • A particle model of light is necessary to describe phenomena observed in modern physics, for example, the interaction between light and atoms. Light as a Particle

2. The Photoelectric Effect • Many physicists’ work contributed to the discovery of the photoelectric effect • What is it? • The ability of light to dislodge electrons from a metallic surface • The electrons can be detected and the resulting signals amplified • Lots of applications in visual imaging Light as a Particle

3. Questions • How many electrons are ejected in a given time? • How does this number depend of wavelength or intensity? • How energetic are the ejected electrons? • Upon what does the electron energy depend? • Are electrons ejected instantly or is there a time delay? Light as a Particle

4. Photoelectric Experiments • Cathode – electrons are ejected • Anode – electrons are collected Experiment 2 Experiment 1 Light as a Particle

5. Photoelectric Experiments - con’t • a) Electrons freely flow from the anode back to the cathode and they are counted along the way • Can determine how # of e- depends on wavelength and intensity; time light must shine on cathode for electrons to flow Light as a Particle

6. Photoelectric Experiments - con’t • b) Ejected electrons have to overcome the electric field to get to the anode Light as a Particle

7. Photoelectric Experiments - con’t • b) Ejected electrons have to overcome the electric field to get to the anode • Can determine energy of ejected electron • If the potential difference between the plates, ΔΦ = 2.0 V, the difference between the electron’s electrostatic potential energy at the anode and its potential energy at the cathode is • The electron can make it to the anode only if it has an initial kinetic energy greater than this Light as a Particle

8. Wave Model Predictions • The rate at which electrons are ejected from a metal is proportional to the intensity of the incident light. • Lower intensity light rays should have a delay before electrons are ejected • The rate may depend on frequency (wavelength) of light • The maximum kinetic energy of the electrons is likely to increase with increasing intensity Light as a Particle

9. Experiments Provide the Following Results • At high intensities and fixed frequencies, the # of ejected electrons is proportional to intensity • Electrons are ejected instantly, regardless of intensity level • For constant intensity, the # of electrons decreases with increasing frequency • If the frequency is below a certain level, no electrons are ejected, regardless of intensity level • Above the cutoff frequency, the electrons’ maximum kinetic energy is proportional to the frequency of light ✔ ✔ ✔ ✔ ✔ Light as a Particle

10. Maximum Energy depends on Frequency • Above the cutoff frequency, the electrons’ maximum kinetic energy is proportional to the frequency of light Light as a Particle

11. Einstein’s Prediction – light is a particle • Light consists of particles, each carrying a certain amount of energy • Where E is the energy, f is the frequency, and h is Planck’s constant • We typically express colors of light in wavelengths Light as a Particle

12. Einstein’s Prediction - con’t • Einstein also predicted that each electron ejected from the metal was a result of a collision with a single photon • Where K is the kinetic energy of the electron and W is the work function for the metal • The work function is the energy required to liberate the electron from the metal Light as a Particle

13. Einstein’s Prediction - con’t • Einstein’s model explains the experimental results so neatly, why was there resistance in the science community? • This model is completely inconsistent with the wave nature of light. • Neither model, wave or particle, adequately explains light by itself Light as a Particle

14. Practice • Interactive activity – photoelectric effect for different metals • http://phet.colorado.edu/simulations/sims.php?sim=Photoelectric_Effect • Group Problems • Q3B.5, Q3S.4 Light as a Particle