Transcript Document
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. PHYS140 Light as a Particle 1 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 PHYS140 Light as a Particle 2 Questions • • • • • How many electrons are ejected in a given time? How does this number depend of wavelength or intensity? How energetic of the ejected electrons? Upon what does the electron energy depend? Are electrons ejected instantly or is there a time delay? PHYS140 Light as a Particle 3 Photoelectric Experiments Experiment 2 Experiment 1 • • Cathode – electrons are ejected Anode – electrons are collected PHYS140 Light as a Particle 4 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 PHYS140 Light as a Particle 5 Photoelectric Experiments - con’t • b) Ejected electrons have to overcome the electric field to get to the anode PHYS140 Light as a Particle 6 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 q 1.6 1019 C2.0V 3.2 1019 J • The electron can make it to the anode only if it has an initial kinetic energy greater than this PHYS140 Light as a Particle 7 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 PHYS140 Light as a Particle 8 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 propostional to the frequency of light PHYS140 Light as a Particle ✔ ✔ ✔ ✔ ✔ 9 Maximum Energy depends on Frequency • Above the cutoff frequency, the electrons’ maximum kinetic energy is proportional to the frequency of light PHYS140 Light as a Particle 10 Einstein’s Prediction – light is a particle • Light consists of particles, each carrying a certain amount of energy E hf • hc Where E is the energy, f is the frequency, and h is Planck’s constant h 6.63 10 34 J s 4.15 10 15 eV s • We typically express colors of light in wavelengths PHYS140 f c hc 1240eV nm Light as a Particle 11 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 K • hc W Where K is the kinetic energy of the electron and W is the work function for the metal • PHYS140 The work function is the energy required to liberate the electron from the metal Light as a Particle 12 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 PHYS140 Light as a Particle 13 Extra Credit • • • • • • Several extra credit projects have been added to the schedule. Check Moodle and the online schedule for more info. Acousto-magnetic strips deter shoplifters, due 4/23 Blue Man Group Pipe Instruments, due 4/23 Make an instrument, due 4/28 Applications of the photoelectric effect, due 4/28 PHYS140 Light as a Particle 14 Practice • Interactive activity – photoelectric effect for different metals • • • Interactive problem • • Go to the site below and answer the questions at the bottom of the page http://www.loncapa.org/~mmp/kap28/PhotoEffect/photo.htm http://wug.physics.uiuc.edu/cgi/courses/shell/per/p hys102/ie.pl?12/pe1 Group Problems • PHYS140 Q3B.5, Q3S.4 Light as a Particle 15