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Phy107Lect23 - From Last Time Light shows both particle and...

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Mon. Nov 1 Phy107 Lecture 23 From Last Time… Light shows both particle and wavelike properties Interference is an example of wavelike props. Photons are particles of light in the particle-like description. Experiments that demonstrated this are Photoelectric effect Blackbody radiation
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Mon. Nov 1 Phy107 Lecture 23 Hour exam 2 results Average = 68% Median = 70% Your grade is posted at [email protected] • uwmad.courses.wisconsin.edu Use your netid and password, then go to Physics 107 course page.
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Mon. Nov 1 Phy107 Lecture 23 Blackbody radiation Blackbody radiation Object emits light with a blackbody spectrum. Peak position (color) changes with temperature White hot Red hot
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Mon. Nov 1 Phy107 Lecture 23 Photoelectric effect: PARTICLES Einstein says that light is made up of photons, individual ‘particles’, each with energy hf . One photon collides with one electron - knocks it out of metal. If photon doesn’t have enough energy, cannot knock electron out. Intensity ( = # photons / sec) doesn’t change this. Minimum frequency (maximum wavelength) required to eject electron
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Mon. Nov 1 Phy107 Lecture 23 …explained by quantization of light Light comprised of discrete particles (photons) Each photon has energy hf ( h = Planck’s constant)
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Mon. Nov 1 Phy107 Lecture 23 Intensity of light Photon of frequency f has energy hf Red light made of only red photons The intensity of the beam can be increased by increasing the photon flux. Interaction with matter Photons are absorbed one at a time Interaction depends on individual photon frequency / energy.
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Mon. Nov 1 Phy107 Lecture 23 Photons and Electromagnetic Waves Light has a dual nature. It exhibits both wave and particle characteristics Applies to all electromagnetic radiation The photoelectric effect gives evidence for the particle nature of light Light can behave as if it were composed of particles Interference and diffraction evidence the wave nature of light
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Mon. Nov 1 Phy107 Lecture 23 Matter waves If light waves have particle-like properties, matter should have wave properties. de Broglie postulated that the wavelength of matter is related to momentum as This is called the de Broglie wavelength. λ = h p
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Mon. Nov 1 Phy107 Lecture 23 Why h / p ? Works for photons Wave interpretation of light: wavelength = (Speed of Light) / Frequency λ = c / f Particle interpretation of light (photons): Energy = (Planck’s constant) x Frequency E = hf, so f = E / h So for a photon
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