馬文蔚,英語講課,10-Base of Quantum theory(2009V1)

1、單擊此處編輯母版標(biāo)題樣式單擊此處編輯母版文本樣式第二級第三級第四級第五級*單擊此處編輯母版標(biāo)題樣式單擊此處編輯母版文本樣式第二級第三級第四級第五級*Chapter10 Base of Quantum theory - Huang QiongMichelson-Morley experiment Blackbody radiation Photoelectric EffectAtomic Spectrum of Hydrogen Compton Effect Quantum mechanicSpecial RelativityDark Clouds in Stretches of Blue Sk

2、y (1899) 1 Blackbody Radiation 1. Blackbody Radiation The body that only absorbs incident electromagnetic wave and doesnt reflect is called the blackbody. The blackbody is an ideal model. the small hole is just approximately a blackbody. Device to Measure the Distribution of Energy in Blackbody Radi

3、ation:a cavity with small hole, the temperature in the cavity is controlled. lensethe light beam radiated from the hole a parallel light tube a disperse prism a thermocouple monochromatic lightTwo Features of the Distribution Intensity : the monochromatic energy radiated from unit area on the surfac

4、e of the black body for unit time. Intensity The amount of radiation emitted (the area under a curve) increases (Stefans law) The peak in the distribution shifts toward shorter wavelengths. (Wiens law) Intensity The energy of different wavelengths radiated from unit area on the surface of the black

5、body for unit time conforms to the equation in the following:Stefan-Boltzmann lawIf represents the wavelength corresponding to the greatest monochromatic energy flux density Intensity Wien Displacement LawWith the increase of temperature the value of moves left.What is the expression equation confor

6、ming to the follow figure? This is a hotspot.0 1 2 3 4 5 6(m)1700K1500K1300K1100KAny attempts to use classical ideas to explain the shapes of the curve is failed.Experimental dataWien formula Rayleigh-Jeans formula Ultraviolet catastrophe infinite energyPlanck Formula After repeated thought Planck b

7、elieves that, the key of solving that question is to recognize the energy is quantized .Where n is a positive integer called a quantum number, h is Plancks constant.The key point of Plancks theory is the radical assumption of quantized energy states.Energy01234n0Planck Formula of Blackbody Radiation

8、 For lower frequency there is Rayleigh-Jeans formulaFor higher frequency there is Wien formulaExperiment dataRayleigh-Jeans formulaWien formulaT=1646kPlancks theoretical dataAt all wavelengths Planck equation for the wavelength distribution that is in complete agreement with the experimental results

9、.Planck introduced the concept of a “quantum of action” in an attempt to explain the spectral distribution of blackbody radiation, which laid the foundations for quantum theory.In 1918,he was awarded the Nobel Prize in physics for the discovery of the quantized nature of energy.(1858-1947) The absor

10、bed or radiates energy is quantized . This development marked the birth of the quantum theory.When Planck presented his theory, most scientist (include Planck!) did not consider the quantum concept realistic. It was felt to be a mathematic trick that happen to predict the correct results. Subsequent

11、 developments ,however, showed that a theory based on the quantum concept (rather than on classical concepts) was required a number of other phenomena at the atom level.在十年之后，1915年，不相信光量子的米立肯(18681953年)宣布 他的實(shí)驗(yàn)無歧義地證實(shí)了愛因斯坦的光電效應(yīng)理論和1922年康普頓(18921962年)發(fā) 現(xiàn)射線散射效應(yīng)必須由光量子論解釋之后，人們才正確評價(jià)了光量子論，宣布愛因 斯坦由于“在理論物理學(xué)方面的

12、成就，特別是光電效應(yīng)定律的發(fā)現(xiàn)”而授予他1921年 度的諾貝爾物理學(xué)獎(jiǎng)。 愛因斯坦和普朗克不同，當(dāng)時(shí)就堅(jiān)信自己的光量子論是“非常革命的”。的確， 光量子論并不是簡單地復(fù)活光微粒說，而是揭示了光的波粒二象性。對統(tǒng)計(jì)平均現(xiàn)象 光表現(xiàn)為波動，對瞬時(shí)漲落現(xiàn)象光表現(xiàn)為粒子。光量子論第一次確認(rèn)了光的波粒二象 性這個(gè)最基本的性質(zhì)。 1. Photoelectric Effect metalExperiments show that light incident on certain metallic surfaces causes electrons to be emitted from the surfa

13、ces. This phenomenon is called the photoelectric effect, and the electron is called the photoelectron. 2 Photoelectric EffectOOOOOOOOLight A Schematic Diagram of a Photoelectric Effect Apparatus.When the evacuated glass tube is kept in dark, the ammeter reads zero, indicating that no current is flow

14、ing in the circuit.When light of the appropriate wavelength shines on plate E, however, a current is detected by the ammeter, indicating a flow of charges across the gap between A and K.High intensityLow intensitycurrentStopping potentialApplied potential differencePhotoelectric current versus appli

15、ed potential difference for two light intensity.The current increases with intensity but reaches a saturation level for large values.2. Experimental law of photoelectric effect and difficulty of classical theory Independence of photoelectron kinetic energy on light intensity.High intensityLow intens

16、itycurrentStopping potentialThis is shown in figure by the fact that both curves fall to zero at the same negative voltage.According to classical prediction, electrons should absorb energy continuously from the electromagnetic waves. A more intensity light should transfer more kinetic energy to the

17、electrons, so the stopping potential should be larger .High intensityLow intensityCurrent Stopping potentialSuppose the initial kinetic energy of photoelectron escaped from the plate K is The lowest voltage to make the current drops just to zero between two poles is is called the restricted voltage,

18、 which makes the photoelectron escaped from the pole K could not just arrive at the pole L. restricted voltage (Stopping potential)v 0CSKCUv01v02v03frequencyStopping potential Dependence of photoelectron kinetic energy on light frequency. The initial kinetic energy of photoelectron is related with t

19、he frequency of incident light instead of the intensity. According to the classical theory, the greater intensity of incident light is, the greater initial kinetic energy of photoelectron. The initial kinetic energy of photoelectron should be related with the intensity of incident light instead of t

20、he frequency. Dependence of ejection of electrons on light frequencyAccording to the classical theory, no matter how many the frequency of incident light is, if only the intensity is great enough, the electron can obtain enough energy to escape from the metal, existing no the cutoff frequency. For o

21、ne kind of metal there exists lowest frequency , which is called the cutoff frequency. If the frequency of incident light is smaller than the cutoff frequency, no matter how many the intensity of incident light is, the photoelectric effect doesnt appear. 0 Time between incidence of light and ejectio

22、n of photoelectronsIf only the frequency of incident light is greater than the cutoff frequency, photoelectrons escaped rapidly, the hysteretic time doesnt exist. According to the classical theory, from the start of shining to pole K to the escape of photoelectrons there should exist the longer or s

23、horter hysteretic time in which the electron absorbs energy of incident light. t(s) 0 10-9I3. Einstein Photoelectron Theory (in 1905) Einstein extended Plancks concept of quantization to electromagnetic waves. He assumed that light of frequency can be considered to be a stream of quanta which is cal

24、led photons. Each of the photons has an energy E given by the following equations : E=hEach photon moves with a velocity C .Einstein Photoelectric Equation the initial kinetic energy the escape work of the meta Dependence of ejection of electrons on light frequency The cutoff frequency is Dependence

25、 of photoelectron kinetic energy on light frequency.v 0CSKCUEinstein received the 1921 Nobel Prize in Physics for his services to Theoretical Physics, and especially for his discovery of the law of the photoelectric effect.3 Compton Effect American physicist Compton (A.H. Compton, 1892-1962) started

26、 to research X ray(10-10m) at himself youth. 1896, Roentgen He discovered in experiment that, after through the graphite the wavelength of some scattered waves doesnt change, and the wavelength of another scattered waves increases, the increment of wavelength increases with the increase of scatterin

27、g angle. This is called Compton effect .After exploration for many years he published the article in May 1923, which claims that experimental phenomenon could be explained only by use of the quantum theory. X-ray sourcediaphragmCarbon targetIonization chamber.(a)(b)(c)(d) 1.Two peaks:0X-rayYXYX If t

28、he electron collided with the photon is in external layer of atom, since the bound action to the electron in external layer is weaker, the electron may be considered as a free electron. So the momentum of the photon is changed after the collision, namely the wavelength of light is increased. But if

29、the electron is in internal layer of atom, since the bound action to the electron in internal layer is stranger, the collision of the photon with the electron matches up to the collision with whole atom, and the mass of atom is greater far than the mass of photon, so the momentum of the photon doesn

30、t change after the collision, namely the wavelength of light doesnt change. Thus, by use of the energy conservation law and the momentum conservation law explained perfectly Compton Effect. Namely, the energy conservation law and the momentum conservation law are both strict tenable in microscopic w

31、orld. Compton wavelength of electron: Compton shift equation:Compton effect is only detectable for wavelengths that are very short.4 Bohr Theory of Hydrogen Atom1. Atomic Spectrum of HydrogenThe atomic spectrum of hydrogen is not continuous.Rydberg formula reflects really the atomic spectrum law of

32、hydrogen. Rydberg formula:Atomic Spectrum of HydrogenLyman series Balmer series Paschen series Brackett series continuous n=4n=3n=2n=1r =a1r =4a1r =9a1r =16a1Lyman series Balmer seriesPaschen series The orbit, velocity, energy of electron are quantized.Niels Hendrik David Bohr (18851962) received the 1922 Nobel Prize in Physics. 2. Bohr Atomic Theory of Hydrogen Bohr guesses that orbit of electron out of nucleus is not arbitrary, and should satisfy certain condition. According to this train of thought Bohr put forward two suppositions in 1913.The orbit of electron out of nucleus is not a