Chapter. 3 Wave & Particles I

Transcription

1 Announcement Course webpage Textbook PHYS-2402 Lecture 3 Sep. 15, 2015 Quiz.1 Thursday [Ch.2] Lecture Notes, HW Assignments, Physics Colloquium, etc.. Chapter. 3 Wave & Particles I M- Waves behaving like Particles Newton(1704): light as a stream of particles. Descartes (1637), Huygens, Young, Fresnel (1821), Maxwell: by mid-19 th century,the wave nature of light was established (interference and diffraction, transverse nature of M-waves). Physics of the 19 th century: mostly investigation of light waves Physics of the 20 th century: interaction of light with matter Outline: 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?

2 Newton(1704): light as a stream of particles. Descartes (1637), Huygens, Young, Fresnel (1821), Maxwell: by mid-19th century,the wave nature of light was established (interference and diffraction, transverse nature of M-waves). Physics of the 19th century: mostly investigation of light waves Physics of the 20th century: interaction of light with matter One of the challenges understanding the black body spectrum of thermal radiation Black body: In physics, a black body is an idealized object that absorbs all &M radiation that falls on it. No &M radiation passes through it and none is reflected. Because no light is reflected or transmitted, the object appears black when it is cold. However, a black body emits a temperature-dependent spectrum of light. (see Fig) This thermal radiation from a black body is termed black-body radiation. Black Body Radiation Newton(1704): light as a stream of particles. (Max Planck 1900) mid-19th Descartes (1637), Huygens, Young, Fresnel (1821), Maxwell: by century,the wave nature of light was established (interference and diffraction, transverse nature of M-waves). xperiment shows that as frequency increases, the blackbody spectral energy density reaches a max. then fall off. But, classical theory predicts a divergence!! Do we need a new theory? Physics of the 19th century: mostly investigation of light waves Physics of the 20th century: interaction of light with matter One of the challenges understanding the black body spectrum of thermal radiation Black body: In physics, a black body is an idealized object that absorbs all &M radiation that falls on it. No &M radiation passes through it and none is reflected. Because no light is reflected or transmitted, the object appears black when it is cold. However, a black body emits a temperature-dependent spectrum of light. (see Fig) This thermal radiation from a black body is termed black-body radiation. As the temperature decreases, the peak of the black-body radiation curve moves to lower intensities and longer wavelengths. (More in Appendix C)

3 In 1900, Planck suggested a solution based a revolutionary new idea: mission and absorption of &M radiation by matter has quantum nature: i.e. the energy of a quantum of &M radiation emitted or absorbed by a harmonic oscillator with the frequency f is given by the famous Planck s formula = h f,where h is the Planck s constant Also, in terms of the h ω = 2π f =hω where h = angular frequency 2 h at odds with the classical tradition, where energy was always associated with amplitude, not frequency π J s h J s 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 h ( Planck Constant ) J s Blackbody Radiation: A New Fundamental Constant Plank s spectral energy density is the critical link between temperature and M radiation. Interestingly, although the assumption = nhf might suggest M radiation behaving as an integral number of particles of energy hf, he hesitated at the new frontier - others carried the revolution forward. For the discovery, Plank was awarded the 1918 Nobel prize!! xperimental Fact: = nhf BUT Why should the energy of an lectromagnetic wave be Quantized? (n= integer) No xplanation until 1905 Albert instein The Photoelectric ffect A wave is a Continuous Phenomenon

4 The Photoelectric ffect (Albert instein 1905) The Photoelectric ffect (Albert instein 1905) Albert instein postulated the existence of quanta of light -- s -- which, when absorbed by an electron near the surface of a material, could give the electron enough energy to escape from the material. ven With Very strong light of low frequency metal Phenomenon observed long time before instein, and something very strange was observed: metal Contradicting Classical Wave Physics NO electrons The Photoelectric ffect (Albert instein 1905) Planck s Law ( = nhf) Photoelectric ffect (Threshold frequency) ven With Very-Very weak light intensity, but of high enough frequency lectrons Albert instein proposed: The light is behaving as a collection of particles called s each of them having energy

5 The Photoelectric ffect (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!

6 Also known at that time: Repels electrons! = 380nm To free an electron from the metal, one has to pay a certain amount of energy the Work Function " =?! f max min =? =? U < 2Volts

7 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. c Ke W = h W λ 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. 7 ( Ke + W) λ ( 1.7eV + 5.4eV) m 15 c h= = = ev s 8 Ke W = h W c 3 10 m/ s λ = ev s J / ev = J s 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 λis changed to 240nm? (b) what is the maximum electron speed? (b) (a) c h W c c c " 1 1 # K = h W = h h = hc% & = 1.9eV ' ( λ = e 0 λ1 λ1 λ0 λ1 λ0 2K 2 e 5 Ke = mv e /2 v= = m/ s me