Announcements. Fast => v~c c= the velocity of light

Transcription

1 Announcements 2402 Lab will be started this week Lab manual is available on the course web page HW: Chapter.2 70, 75, 76, 87, 92, 97*, 99, 104, st Quiz: 9/18 (Ch.2) Nonclassical Physics *** Course Web Page ** Lecture Notes, HW Assignments, Schedule for the Physics Colloquium, etc.. Small => e.g. atomic size Quantum mechanics (1920 s-) Relativistic quantum mechanics (1927-) Classical physics Relativistic mechanics, l.-mag. (1905) Fast => v~c c= the velocity of light Next Topic: Our First Topic

2 Outline: Lecture 7 Chapter. 3 Wave & Particles I M- Waves behaving like Particles 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? Newton (1704): light as a stream of particles. Historical Development 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. One of the challenges understanding the blackbody 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. Black Body Radiation (Max Planck 1900) xperiment shows that as frequency increases, the blackbody spectral energy density reaches a max. then fall off. Classical theory predicts a divergence. Historical Development Planck (1900) suggested a solution based a revolutionary new idea: emission and absorption of M radiation by matter has quantum nature: 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 where h is the Planck s constant - at odds with the classical tradition, where energy was always associated with amplitude, not frequency Also, in terms of the angular frequency where (More in Appendix C)

3 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 The Photoelectric ffect A wave is a Continuous Phenomenon 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. RMINDR:: Waves (based on PHYS 1408/2401) Wave equation in one dimension for any quantity : Solution: a plane wave traveling in the negative (positive) direction x with velocity v: v the phase velocity Harmonic plane wave traveling in the positive direction x: angular frequency wave number A 0 -A 0 A 0 t = 0 t = t 0 vt 0 x x For the discovery, Plank was awarded the 1918 Nobel prize lectromagnetic waves: (transverse in free space) -A 0 T t

4 Photoelectric ffect Historical Note: The photoelectric effect was accidentally discovered by Heinrich Hertz in 1887 during the course of the experiment that discovered radio waves. Hertz died (at age 36) before the first Nobel Prize was awarded. The Photoelectric ffect (Albert instein 1905) Observation: when a negatively charged body was illuminated with UV light, its charge was diminished. J.J. Thomson and P. Lenard determined the ratio e/m for the particles emitted by the body under illumination the same as for electrons. The effect remained unexplained until 1905 when 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. Robert Milliken carried out a careful set of experiments, extending over ten years, that verified the predictions of instein s theory of light. instein was awarded the 1921 Nobel Prize in physics: "For his services to Theoretical Physics, and especially for his discovery of the law of the photoelectric effect." Milliken received the Prize in 1923 for his work on the elementary charge of electricity (the oil drop experiment) and on the photoelectric effect. metal Phenomenon observed long time before instein, and something very strange was observed: The Photoelectric ffect (Albert instein 1905) The Photoelectric ffect (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

5 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 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

6 xample (2): SINGL PHOTON Very weak light beam of high frequency Photon-based explanation of Ph.. Absorption of a by an electron in metal (inelastic collision between these particles) before However, we ve concluded that a free electron cannot absorb a after beam = = nhf LARG (above the threshold) before after the rest RF of an electron after the collision What s wrong? The electron is not free, it is embedded in metal, and the chunk of metal is the second body that participates in the collision 1 electron energy conservation momentum conservation The PHOTON ejects 1 LCTRON Thus, while the electron is still inside metal energy conservation momentum conservation The energy is absorbed by an electron (the energy absorbed by metal is negligibly small), but the momentum exchange between electron and metal is crucial for momentum conservation. Photon-based explanation of Ph.. (cont d) In the experiment, the electron is observed outside the metal. It takes some energy to escape: (consider an attraction between an electron and the positive image charge induced on the metal surface) The escape energy: the work function " (material-specific) q + metal q - nergy Conservation: "1# "2# Thus, for the electron outside metal "# red boundary of Ph.. "# "# = hf K max " Planck s constant measurements: "#

7 = 380nm Repels electrons U < 2Volts " =? f max min =? =?