Announcements. Lecture 6 Chapter. 3 Wave & Particles I. Experimental Fact: E = nhf. EM- Waves behaving like Particles

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1 Announcements HW2: Ch.2-70, 75, 76, 87, 92, 97, 99, 104, 111 HW2 due: 2/9 (by class hour) ** Lab manual is posted on the course web *** Course Web Page *** Lecture Notes, HW Assignments, Schedule for thephysics Colloquium, etc.. Outline: Lecture 6 Chapter. 3 Wave & Particles I M- Waves behaving like Particles! Blackbody Radiation (Plank; 1900; 1918*)! (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? The Planck s Black-Body Radiation Law: The nergy () in the electromagnetic radiation at a given frequency (f) may take on values restricted to = nhf where: n = an integer h = a constant ( Planck Constant ) xperimental Fact: = nhf BUT Why should the energy of an lectromagnetic wave be Quantized? (n= integer) No xplanation until 1905 Albert instein A wave is a Continuous Phenomenon

2 (Albert instein 1905) (Albert instein 1905) ven With Very strong light of low frequency metal Contradicting Classical Wave Physics NO electrons ven With Very-Very weak light intensity, but of high enough frequency lectrons Also known at that time: Planck s Law ( = nhf) Photoelectric ffect (Threshold frequency) Albert instein proposed: To free an electron from the metal, one has to pay a certain amount of energy the Work Function The light is behaving as a collection of particles called s each of them having energy

3 (Albert instein 1905) xample (1): Very intensive light beam, low frequency light ven With Very-Very weak light intensity, beam but of high enough frequency = nhf lectrons What happens is that 1 PHOTON ejects 1 LCTRON beam = nhf SMALL (below the threshold) LARG (n is large) There is no PHOTON capable of ejecting an LCTRON NO lectrons xample (2): SINGL PHOTON Very weak light beam of high frequency nergy Conservation: beam = = nhf LARG (above the threshold) 1 electron = hf K max "! The PHOTON ejects 1 LCTRON!

4 ! = 380nm Repels electrons U < 2Volts " =?! f max min =? =? 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. 2. The threshold wavelength for emission of electrons from a given metal surface is 380nm. (a)! what will be the max kinetic energy of electrons when! is changed to 240nm? (b)! what is the maximum electron speed?

5 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. 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 2. The threshold wavelength for emission of electrons from a given metal surface is 380nm. (a)! what will be the max kinetic energy of electrons when! is changed to 240nm? (b)! what is the maximum electron speed? (a) (b) X-rays can be produced by smashing highspeed electrons into a metal target. When they hit, these decelerating charge produce much radiation, called Bremsstrahlung The Production of X-Rays (Wilhelm Roentgen 1901) SURPRIS: xperiments indicate a cutoff wavelength: (The reverse of the Photoelectric ffect) There is no classical explanation for so sharp a termination of the spectrum Frequency f, nergy =hf CLASSICAL physics: Radiation covers entire spectrum Bremsstrahlung 1 -> 1 electron (?) 1 electron -> 1 (?)

6 SURPRIS: xperiments indicate a cutoff wavelength: 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. Frequency f INDD: Setting the K of an incoming electron = of one Frequency f INDD: 1 electron -> 1 The Compton effect (Arthur Compton 1927) Momentum & nergy when a strike a free electron Is this true? Compton provided the 1 st experimental evidence!! momentum energy Hypothesis: xperiment? Before Collision: A approaches an electron at rest

7 Momentum & nergy when a strike a free electron Momentum & nergy when a strike a free electron After Collision: The electron scatters at speed u, angle ". A of wavelength! scatters at angle # nergy and Momentum Conservation X-ray detector 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 -