71Particlenatureoflight - 6.1 Quantum Physics Particle...

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1 6.1 Quantum Physics. Particle Nature of Light •Particle nature of Light Blackbody Radiation Photoelectric Effect • Properties of photons Ionizing radiation Radiation damage • x-rays Compton effect X-ray diffraction Photons When light exchanges energy with atoms it behaves as a particle - called the photon The energy of a photon is proportional to the frequency f of light E photon = hf Planck’s Constant h=6.626x10 -34 J • s Thermal Radiation Blackbody radiation A container at temperature T in equilibrium with electromagnetic radiation. Light is absorbed and emitted by the walls. At equilibrium the spectrum of the light only depends on the temperature. Spectrum of Blackbody radiation A wide spectrum of wavelengths is produced. The total intensity increases with temperature The peak wavelength decreases with temperature Wien’s displacement law λ max T= 0.2898x10 -2 mK The intensity goes down at low wavelengths (I->0, as λ -> 0) infrared visible Demonstration of blackbody radiation A tungsten filament light bulb is approximately a black body radiator. http://www.physics.ucla.edu/demoweb/demomanual/astronom y/quantum_mechanics/blackbody_radiation.html Disagreement with classical theory of light The classical theory predicts that intensity continues to increase with decreasing wavelength. “Ultraviolet Catastrophe” To explain the experimental data Planck proposed the quantum hypothesis.
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2 Planck’s constant Planck proposed that light could only have certain energies E=hf Then the energy of oscillators in the black body could only have certain fixed values Max Planck Classical and Planck picture Suppose we have a box that contains light waves with different wavelengths. The energy is contained in energy states containing particles with different energies Classical theory predicts that the number of energy states (like standing waves) increased with decreasing wavelength. Planck proposed that in addition the short wavelength particles are more “energetically expensive” So at short wavelength, they would be hard to produce. This explains the peak in the black body spectrum Quantum explanation for the Wein Effect.
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71Particlenatureoflight - 6.1 Quantum Physics Particle...

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