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equipment 2. Photoelectric Effect. Einstein’s Photon Theory ( 光电效应、爱因斯坦光子理论 ) R K1K1 K2K2 E G V les, called photons. Einstein assumed that the energy in a light beam travels through space in concentrated bund The energy E of a single photon is given by E = hv. Applying the photon concept to the photoelectric effect, Einstein wrote 光电管 K A

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Equation above says that a photon carries an energy hv into the sur- face. Part of this energy A 0 is used in causing the electron to pass through the metal surface. The excess energy (hv-A 0 ) is given to the electron in the form of kinetic energy. In 1916, Milikan, experiments verified Einstein’s ideas in every detail. Einstein succeed in explaining the photoelectric effect. h - - 金属金属 electron - - - A

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The dual nature of light, particle and wave 光的波粒二象性 particle wave （ energy ） （ frequency ） （ momentum ） （ wavelength ） h These two natures are connected by h.

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3. The Compton effect ( 康普顿效应 ) Compelling confirmation of the concept of the photon as a concen- trated bundle of energy was provided in 1923 by A.H.Compton who earned a Noble prize for this work in 1927. Compton allowed a beam of X-rays of sharply defined wavelengthλto fall on a graphite block. He measured, for various angles of scattering, the intensity of the scattered X-rays as a function of their wavelength.

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1)Equipment Compton allowed a beam of X-rays of sharply defined wavelengthλto fall on a graphite block. He measured, for various angles of scattering, the intensity of the scattered X-rays as a function of their wavelength. X光X光 光栏光栏 石墨 0.70Å X 射线分析仪

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1) Equipment 2) Experimental results : X光X光 光栏光栏 石墨 0.70Å X 射线分析仪 2 3 4 1 =0 0 =45 0 =90 0 =135 0 0.700.75 (Å) 强度强度 The scattered X-rays have intensity peaks at two wavelengths: one of the them is the same as the incident wavelength, 0, the other,, being larger by an amount △.

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2 3 4 1 =0 0 =45 0 =90 0 =135 0 0.700.75 (Å) 强度强度 This so-called Compton shift △ varies with the angle at which the scattered X-rays are observed. Compton shift △ for collisions with tightly bound electrons is immeasurably small.

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3) Explain Compton Effect 对康普顿效应的解释 Compton was able to explain his experimental results by postulation that the incoming X-ray beam was not a wave but an assembly of photons of energy E (= hv) and that these photons expe- rienced billiard-ball-like collisions with the free electrons in the scattering block. h 0 m0m0 e X BeforeAfter h 0 e m V h

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X Analyze quantitatively m0m0 e h 0 m0m0 e BeforeAfter Before After ElectronPhotonElectronPhoton energy momentum 0 Let us assume this is an elastic scattering collision. =++ = + h 0 e m V h

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Analyze quantitatively BeforeAfter { From （ 1 ） From the low of cosine X m0m0 e h 0 m0m0 e e m V h

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Analyze quantitatively （3）2-（5）（3）2-（5） BeforeAfter X m0m0 e h 0 m0m0 e e m V h

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（ 3 ） 2 - （ 5 ）： or

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（m）（m） （Å）（Å） ….(10) ….(11) Compton wavelength

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（Å）（Å） （m）（m） ….(10) ….(11) Discuss ， Equation (11) predicts within experimental error the experimentally observed Compton shifts. The peak for 0 can be understood as resulting

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Discuss ， The peak for 0 can be understood as resulting from a collision between a photon and electrons bound in an ionic core. The electron’s effective mass is much greater. ionic core 铅球 Photon 乒乓球 The Compton effect for longer wavelength is hardly observed. quantum results reduces to classical results. For example, 0 = 10 cm, △ / 0 = 2.43×10 -11, the Compton effect is hardly observed.

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4 ） Significance A ） Compelling confirmation of the concept of the photon as a concentrated bundle of energy was provided. The photon has mass, energy and momentum. B) In microscopic field, the law of conservation of momentum and the law of conservation of energy are still strictly observed. Exp. An X-ray with E = 0.60MeV are scattered from a carbon block. Its wavelength changed 20% in the collision. What kinetic energy is imparted to the recoiling electron?

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例：能量为 0.60MeV 的 X 射线在碳块上散射后，波 长变化了 20% ，求反冲电子动能。 Known, E 0 =h 0 =0.6MeV, =0.2 0, Find, E e =? Solve, kinetic energy of the recoiling electron, m0m0 e h 0 e m V h

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kinetic energy of the recoiling electron, The kinetic energy 0.10Mev imparted to the recoiling electron. h 0 e m V h

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