浙江大学大学物理甲下chapter(1).ppt

1,Chapter 45 The Nature of Light,2,45-1 Introducing the Photon,Interference, diffraction wave-like,Photon: proposed by Einstein in 1905,Black-body radiation, Photoelectric Effect, Compton Effect Particle-like,3,Light is neither a particle nor a wave. Light is both a particle and a wave.,“particle” and “wave”, these two descriptions are clearly inconsistent with one another. Is light a particle or a wave?,4,45-2 Thermal Radiation,Thermal radiation: radiation emitted by a body because of its temperature.,Two important properties of thermal radiation.,Cavity radiation:,The radiation emerging from the hole should not depend on the material or the mode of construction of cavity but only on the temperature.,5,Apparatus measuring the radiancy R() of the black body,R(),6,Stefan-Boltzmann lawThe total radiated power for all wavelength per area:,Wien displacement law:,With a m of 500 nm for sun radiation, the temperature of sun:,7,I(T) :total power radiated at all wavelengths per unit surface area. R(,T) is spectral radiancy.,Find the function R(,T) with .,Wiens formula,Rayleigh-Jeans formula,Rayleigh-Jeans formula,8,Plancks radiation law (1900),If is small,If is large,9,Max Planck assumed that the atoms that formed the walls of the cavity behaved like tiny harmonic oscillate and the energy of harmonic oscillator is quantized. E = hf h=6.626 10-34 Joule sec,the beginning of the full development of quantum physics.,10,45-3 The Photoelectric Effect,Problems facing to classical physics:,11,1. Intensity problem,Brighter light causes an increase in current (more electrons ejected) but does not causes the individual electrons to gain higher energies. In other words, there is a stopping potential (the maximum kinetic energy of the electrons), its value is does not depend on the intensity of the light.,Stopping potential V0,12,2. Frequency problem,V0,f0,If the frequency of the incident light is below the threshold, no electrons are emitted no matter what the intensity of the incident light.,The maximum kinetic energy of the emitted electrons does depend on the frequency of the incident radiation. There is a threshold frequency f0 (cutoff frequency). The frequency of incident light must be greater than this value f0.,Cutoff frequency f0,f0,f,13,3. Time delay problem When incident light falls on the metal, electrons are emitted virtually instantaneously; the time delay observed experimentally is about 10-9s, regardless of the light intensity.,14,Einsteins analysis of the photoelectric effect,Light quantum (later comes photon) was first introduced by Einstain.,The title of the paper published 1905 is “On a Heuristic Viewpoint Concerning the Production and Transformation of Light”(关于光的产生和转化的一个试探性观点 ),15,Stopping potential,Intensity of light , the number of photon , but the energy of a photon is the same.,Light of greater intensity (but constant frequency) delivers more photons per unit time to the metal surface, so the number of electrons ejected per second increases. However the energy of each photon remains the same. The maximum kinetic energy of the emitted electrons does not depend on the number of photons striking the metal per second because each emitted electron is the result of the absorption of one photon.,16,Cut off frequency,Relation between stopping potential and frequency:,Higher-frequency light has large energy photons. This energy can potentially become the electrons kinetic energy. Thus Kmax increases with increasing frequency. Below the threshold frequency, a photon does not have enough energy to free an electron from the metal, so no electrons are emitted.,17,Time delay problem A single photon carries an energy hf and emits to metal surface, where it is transferred to a single electron. At low intensities, the number of photons per second is small, but the energy is still delivered in discrete packets. Just after the light is turned on, some photons hit the surface: some of them are absorbed and eject electrons from the metal. There is no time delay because the electrons cannot gradually accumulate energy; they either absorb a photon or they dont.,18,Einstein was awarded the Nobel Prize in 1921 “for his services to theoretical physics, and especially for his discovery of the law of the photoelectric effect.”,However,19,45-4 The Compton Effect,intensity,The scattered x ray have intensity peaks at two wavelengths, varies with the angle .,20,Photon is already particle:,Scattering by electron:,21, does not depend on the wavelength 0 of the incident radiation.,The intensity peak, which wavelength does not change on scattering, is resulted by interaction of incoming photon and an electron tightly bound to an atom of target.,22,Wave feature of light:,Photonquantum of light:,Summarize:,23,45-5 The Photon Revealed,An experiment that not only supports the photon concept but that cannot also be explained by the wave concept.,Early experiments (1974), due to the small fluctuations, it is not yet a convincing test for the presence or absence of photons,24,In 1986, P. Grangier et al made chief modification for source S:,The experiment worked brilliantly.,25,45-6 Photons and Waves,Showing waves and showing photons,What is the nature of the light in transmit through the apparatus of Fig.45-12, after it leaves beam splitter B1 but before it reaches beam splitter B2?,Does the light know whether or not beam splitter B2 is in place before it reaches the location of B2?,26,Wave-particle dualism of light,Light is both a particle and a wave. Light is neither a particle nor a wave.,How can we mix wave and particle aspects of light into a consistent description?,The particle and wave natures of light are intimately connected, and we can force it to reveal either type of behavior according to the kind of experiment we choose to perform.,27,Quick quiz A very weak light source, which emits only one photon at a time, is allowed to emit light through a double slit. The light then shines on a photographic film. What will be seen on the film if it is exposed for only a very short period of time? (1) The film will show only discrete spots where photons struck the film, with no evidence of an interference pattern. (2) The film will show only discrete spots where photons struck the film, but the spots would be distributed similar to an interference pattern. (3) The film would show a continuous interference pattern.,28,45-7 Slowing