大学物理教学课件：25 Early quantum physics and the photon

1、Early quantum physics and the photon Chapter 25 25.1 Quantization 25.2 Blackbody radiation 25.3 The photoelectric effect 25.4 Compton scattering 25.6 Spectroscopy of the atom 25.7 The Bohr model of the hydrogen atom 25.5 The wave-particle duality most quantities are continuous 25.1 Quantization poss

2、ible values are limited to a discrete set Quantization: In classical physics: 25.2 Blackbody radiation You can feel the heat from a fireplace without looking at it Black object absorbs radiation more effectively, meanwhile, it radiates as well. Thermograph of a home shows the windows are major sourc

3、es of heat loss. Blackbody: a body absorbs all the radiation it received. The small hole acts approximately as an ideal backbody. rough black surface Blackbody radiation curves 0 e bT m Wien displacement law Kmm2.898 b 0 e -How to measure temperature of molten steel or the sun? Radiation thermometer

4、s are also applied to measure body temperature. The temperature of the skin is approximately 35C. At what wavelength does the radiation emitted from the skin reach its peak? m940 273)K(35 mK100.2898 -2 m T b This radiation is in the infrared region of the spectrum. Total energy under certain T : 4 )

5、(TTE 0 Stephan-Boltzmann law -4-2-8 KWm105.67 0 e 0 00 T)d,(e)(TE What is rate of heat transfer by radiation from and unclothed person standing in a dark room with TS=25C ? How much does a person radiate? The person has a normal T=35C skin temperature and a surface area of A=1.50m2. 765WAA 0 4 )(TTE

6、E 94W)A-( 44 S TTE emit energy to the surroundings get energy from the surroundings A person at rest produce only 125W of body heat. Conduction and convection would also remove heat. Put clothes on! The net rate of radiation energy Rayleigh-Jeans law Experimental points Weins curve Plancks curve 0 e

7、 How to describe blackbody radiation in theory? 1e 1hc2 (T)e Thc/k3 0 The energy of oscillator is discontinuous: Energy changes must occur only in discrete amount called quanta (singular, quantum). fh )1,2,3,(n nhf E Plancks constant h=6.626*10-34Js frequency of the oscillator energy quantization (e

8、nergy level) The Nobel Prize in Physics 1918 in recognition of the services he rendered to the advancement of Physics by his discovery of energy quanta Max Planck 1858-1947, Germany Berlin University When light strikes materials, it can eject electrons from them. 25.3 The photoelectric effect Metal

9、plate 1 1 Collecting wire + I Electrons lose kinetic energy when moving from plate to the wire. Metal plate 11 Collecting wire + I The stopping potential Vs is the magnitude of the potential difference that stops even the most energetic electrons. smax eVK 1) Brighter light causes an increase in cur

10、rent but does not give the individual electrons higher kinetic energies. Metal plate 11 Collecting wire + I 2) The Kmax is independent of the intensity, which only depends on the frequency. Puzzles for classical physics: Light is treated as mechanical wave Metal plate 11 Collecting wire + I 3) There

11、 is a threshold frequency f0 below which no electrons are ejected, regardless of intensity. 4) Once EM radiation falls on a material, electrons are ejected without delay. Puzzles for classical physics: Light is treated as mechanical wave the wave-particle duality of light light quanta ( photon ) hfE

12、 h p energy momentum f, EM radiation is itself quantized. Einsteins photoelectric equation: -hfKmax the energy needed to break the bond between a metal and one of its electrons also called binding energy (BE) -hfKmax h f0 Threshold frequency: 25.4 Compton scattering X-ray source collimator Target (g

13、raphite crystal) detector Both with and scattering wave can be observed. If a photon behaves like a particle, it collide with an electron initially at rest. complete elastic collision energy conservationmomentum conservation complete elastic collision energy conservation momentum conservation EKE e

14、hc K hc e ppp e sinsin coscos h p0 h p h e e vmp vm 2 1 K ee 2 ee 22 ee 2 ee 2 e 2 e cv-1 1 vmp c1)m-(K cmmcE mm / classical expressions relativistic expressions )cos-(1 m h e c Compton shift: Compton wavelength = 0.002426nm compare: exp. value = 0.00241nm The Nobel Prize in Physics 1927 “for his di

15、scovery of the effect named after him Arthur Compton 1892-1962, USA University of Chicago 25.5 The wave-particle duality In 1923, de Broglie suggested wave-particle duality may pertain to particles as electrons and protons as well as to light. fhE h p particle wave The Nobel Prize in Physics 1929 “f

16、or his discovery of the wave nature of electrons de Broglie 1892-1987, France Sorbonne University 25.5 Spectroscopy of the atom Hydrogen spectrum wavelengths can be expressed by Balmer equation: Hydrogen spectrum (discrete spectral lines) where nf =2 and ni = 3,4,5,or 6. The Rydberg constant R=1.097

17、107m-1 is determined by experimental data. ) n 1 - n 1 (R 2 i 2 f 1 25.6 The Bohr model of the hydrogen atom Hydrogen atom in classic description: +e -e r v 2 0 22 e r4 e r v m centripetal force = Coulomb acting force r8 e r4 e vm 2 1 UKE 0 2 0 2 2 e The total energy E(r) can be written as: The valu

18、e of E(r) can vary continuously. r8 e E(r) 0 2 The value of E(r) varies continuously. +e -e r v continuous spectrum The value of r (orbit radius) varies continuously. Bohr Model: (1)The electron can exist without radiating energy only in certain orbits, which called stationary states, and should sat

19、isfy )(1,2,3nn 2 h nL n ( angular momentum quantization). (2) The transition between stationary states leads emission or absorption of a single photon. hfE 2 0 22 e r4 e r v m 2 h nvrmL en vmrL e 0 2 e 2 e 4 rem vr)(m 2 0 2 e ) 2 h (n 4 rem 2 0 2 2 e 2 0 n nan em h r 2 e 2 0 0 em h a )(1,2,3nanr 0 2

20、 n 2 0 2 2 e 2 0 n nan em h r r8 e E(r) 0 2 22 4 e n n 1 h8 em E 2 0 22 4 e n n 1 h8 em E 2 0 Energy quantization ( energy level ) discrete spectral lines orbit radii r discrete angular momentum quantization 2 h nL n 0 2 n anr )( 2 i 2 f nn n 1 n 1 13.6eVEEE fi energy expressed in eV (electron volt)

21、 )( 2 i 2 f nn n 1 n 1 13.6eVEEE fi energy expressed in eV (electron volt) )( 2 0 2 i 2 f 2 4 e nn n 1 n 1 h8 em EEE fi c hhfE )( 2 0 2 i 2 f 3 4 e n 1 n 1 h8 em1 c ) n 1 - n 1 (R 2 i 2 f 1 from exp. data: )( 2 0 2 i 2 f 3 4 e n 1 n 1 h8 em1 c 1.09737107 m-1 by Bohr theory 1.09678107 m-1 by spectroscopic data ) n 1 - n 1 (R 2 i 2 f 1 The Nobel Prize in Physics 1922 “for his services in the investigation of the structure of atoms and of the