Announcements. Lecture 8 Chapter. 3 Wave & Particles I. Albert Einstein proposed: The Photoelectric Effect (Albert Einstein 1905)

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1 Announcements! 2402 Lab (Tuesday/Thursday)! Lab manual is available on the course web page! HW answers are also available now! 1 st Quiz: 9/18 (Ch.2) *** Course Web Page ** Lecture Notes, HW Assignments, Schedule for thephysics Colloquium, etc.. Outline: Lecture 8 Chapter. 3 Wave & Particles I M- Waves behaving like Particles! Blackbody Radiation (Plank; 1900; 1918*)! The Photoelectric ffect (instein; 1905; 1921*)! The Production of X-Rays (Rontgen;1901; 1901*)! The Compton ffect (Compton; 1927; 1927*)! Pair Production (Anderson; 1932; 1936*)! Is It a Wave or a Particle?! Duality? Planck s Law ( = nhf) Photoelectric ffect (Threshold frequency) The Photoelectric ffect (Albert instein 1905) Albert instein proposed: The light is behaving as a collection of particles called s each of them having energy ven With Very-Very weak light intensity, beam but of high enough frequency = nhf lectrons ejected What happens is that 1 PHOTON ejects 1 LCTRON

2 xample (1): Very intensive light beam, low frequency light xample (2): SINGL PHOTON Very weak light beam of high frequency beam = nhf SMALL (below the threshold) LARG (n is large) beam = = nhf LARG (above the threshold) NO lectrons ejected 1 electron ejected There is no PHOTON capable of ejecting an LCTRON The PHOTON ejects 1 LCTRON nergy Conservation: Problems 1. The work function of tungsten surface is 5.4eV. When the surface is illuminated by light of wavelength 175nm, the maximum photoelectron energy is 1.7eV. Find Planck s constant from these data. = hf K max "! 2. The threshold wavelength for emission of electrons from a given metal surface is 380nm. (a)! what will be the max kinetic energy of ejected electrons when! is changed to 240nm? (b)! what is the maximum electron speed? (a)! (b)

3 The Production of X-Rays (Wilhelm Roentgen 1901) (The opposite of the Photoelectric ffect) We use the name X-rays for M radiation whose wavelengths are in the 10-2 nm to 10 nm region of spectrum The Production of X-Rays (Wilhelm Roentgen 1901) (The reverse of the Photoelectric ffect) X-rays can be produced by smashing highspeed electrons into a metal target. When they hit, these decelerating charge produce much radiation, called Bremsstrahlung CLASSICAL physics: Radiation covers entire spectrum Bremsstrahlung SURPRIS: xperiments indicate a cutoff wavelength: SURPRIS: xperiments indicate a cutoff wavelength: Frequency f, nergy =hf There is no classical explanation for so sharp a termination of the spectrum 1 -> 1 electron (?) 1 electron -> 1 (?) Frequency f INDD: If the radiation is quantized, the minimum allowed at f is hf (single ). We can t produce half a, so if multiple electrons don t combine their s into a single, no could be produced of > K of a single electrons. Setting the K of an incoming electron = of one

4 The Compton effect (Arthur Compton 1927) Frequency f INDD: 1 electron -> 1 Hypothesis: Is this true? Compton provided the 1 st experimental evidence!! xperiment? Momentum & nergy when a strike a free electron Momentum & nergy when a strike a free electron momentum energy Before Collision: A approaches an electron at rest After Collision: The electron scatters at speed u, angle ". A of wavelength! scatters at angle #

5 Momentum & nergy when a strike a free electron nergy and Momentum Conservation Problem An X-ray with!=60pm is scattered over by a target electron. (a)! Find the change of its wavelength. (b)! Find the angle between the directions of motion of the recoil electron and the incident. (c)! Find the energy of the recoil electron. X-ray detector (a) The Compton ffect X-ray tube target (light atoms, e.g. graphite) Photons carry momentum like particles and scatter individually with other particles indeed, the wavelength shift is independent of the target material and the initial wavelength. e - (b) (c) momentum conservation: along y axis: along x axis:

6 Problem (a) An X-ray with wavelength 0.8nm is scattered by an electron at rest. After the scattering the electron recoils with a speed equal to 1.4$10 6 m/s (non-relativistic case). NOT: IN TH FOLLOWING, DO TH CALCULATIONS TO AT LAST 4-DCIMAL PLAC ACCURACY m/s (a) Calculate the energy of the scattered in units ev (use conservation of energy). (b) Calculate the Compton shift in the s wavelength, in meters. (c) Calculate the angle through which the was scattered. The photoelectric effect and the Compton effect are two important ways in which M radiation interacts as a particle with matter. We ll discuss a third on Friday! (b) (c)