浙江大学大学物理甲下chapter45.ppt

* Chapter 45 The Nature of Light Chapter 45 The Nature of Light 45-1 Introducing the Photon Interference, diffraction…… wave-like Photon: proposed by Einstein in 1905 Black-body radiation, Photoelectric Effect, Compton Effect….. Particle-like 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? 45-2 Thermal Radiation Thermal radiation: radiation emitted by a body because of its temperature. increasing T 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. Apparatus measuring the radiancy R(λ) of the black body R(λ) Stefan-Boltzmann law—The total radiated power for all wavelength per area: Wien displacement law: With a ?m of 500 nm for sun radiation, the temperature of sun: I(T) :total power radiated at all wavelengths per unit surface area. R(λ,T) is spectral radiancy. Find the function R(λ,T) with λ. Wien’s formula Rayleigh-Jeans’ formula R(λ,T) Rayleigh-Jeans’ formula Wien’s formula Planck’s radiation law (1900) R(λ,T) Planck’s radiation law If λ is small, If λ is large, 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. 45-3 The Photoelectric Effect i Problems facing to classical physics: 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 th