Physics 2DL Lectures Vivek Sharma Lecture # 2 Discussion of Experiments

House Keeping Details  Every one reported to the lab in assigned sections ? Know which experiment you are doing this week ?  Could download pdf or WORD file of experiment ? We switched the web server hardware on you on last friday  But you could not tell (I hope) !  Helpful reading material on E-reserve Modern Physics: Serway, Moses, Moyer Modern Physics : Beiser Resnick-Halliday-Walker : Chapters on Interference & Diffraction  All should be accessible from UCSD computers of (UCSD web proxy) as PDF files  Lectures: Plan Start with a brief overview of Physics behind the experiments Statistical analysis of data + advanced concepts in Probability

Lecture Schedule In Light of My Travel  2D last quarter kept me tied to UCSD, Scientific responsibilities require me to travel to Europe in April & May Ringberg Castle, Bavaria Isola de Elba (Italy) I would like to schedule makeup lectures early in this quarter : Wednesday same time OK with you ?

Additional Lecture this Week + Early Final Exam for 2DL ?  Additional lecture to review experiments this WED same time ? Other dates : Wed 16 Apr, Fri 18 Apr, May 14 th, May 16 th  feedback please  There are 6 labs  (1)+6 Weeks < 10 Weeks   plenty of time  Schedule Final exam (50 minutes, 3 problems) prior to the finals week to get this course out of your way? Tentative Date : 4 th June ?  Pl. keep checking the Class Announcements page at the 2DL web site: for late breaking information (rescheduled lectures, finals etc)

Experiments You Will Do In No Particular Order 1. Optical Spectra & Diffraction Grating 2. Coherence of Light & Interferometry 3. Photoelectric Effect 4. e/m of Electron 5. Franck-Hertz Experiment 6. Electron Diffraction 7. Thermal Band Gap in Solids (optional because neither 2D or 4E will teach you this)

Determination of e/m for Electron  e/m is characteristic of a particle : electron Vs Cl - ion  When Uniform magnetic field of strength B is established perpendicular to direction of motion of a charged particle, particle moves in a circular path of radius R

e/m, e, m of Electron : Why Important Realization that electron is much less massive than the Hydrogen atom made physicists think about the structure Inside atom The electron was discovered just a bit over 100 years ago, triggered A scientific revolution Thomson’s idea Still used to measure Masses of fundamental Particles or nuclei

The Photo Electric Effect: Nobel Prize for Einstein  Maxwell’s Equations  EM Wave Properties  Hertz & Electromagnetic Waves ( Experiment)  Description of Photoelectric Effect Failure of classical physics (why?)  Einstein’s “Quantum” Interpretation inspired by Max Planck  Bottomline : How EM waves propagate Interaction of EM waves with Matter  How to prove that Einstein was right : Measure “h” Find it is same as in Blackbody radiation

Maxwell’s Equations & EM Waves permeabilitypermittivity

EM Waves

Hertz : The EM Wave Guy

PhotoElectric Effect Can tune I, f, Light of intensity I, wavelength and frequency incident on a photo-cathode Measure characteristics of current in the circuit as a fn of I, f,

PhotoElectric Effect

PhotoElectric Effect : E = h f = Quantized

Franck-Hertz Experiment : A prelude Bohr Atom : Discrete orbit  Emission & Absorption line classical Bohr’s quantization

Franck Hertz Experiment: Playing Football ! Inelastic scattering of electrons Confirms Bohr’s Energy quantization Electrons ejected from heated cathode At zero potential are drawn towards the positive grid G. Those passing thru Hole in grid can reach plate P and cause Current in circuit if they have sufficient Kinetic energy to overcome the retarding Potential between G and P Tube contains low pressure gas of stuff! If incoming electron does not have enough energy to transfer =E 2 -E 1 then Elastic scattering, if electron has atleast KE=  then inelastic scattering and the electron does not make it to the plate P  Loss of current

(J) Franck & (G) Hertz Experiment Current decreases because many Electrons lose energy due to inelastic Scattering with the Hg atom in tube And therefore can not overcome the Small retarding potential between G  P The regular spacing of the peaks Indicates that ONLY a certain quantity Of energy can be lost to the Hg atoms =4.9 eV. This interpretation can be confirmed by Observation of radiation of photon energy E=hf=4.9 eV emitted by Hg atom when V 0 > 4.9V