lecture18 101028 - Photons Photoelectric Effect - Example...

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1 Photons Quantum theory describes light as a particle called a photon According to quantum theory , a photon has an energy given by E = hf = hc/ h = 6.6x10 -34 [J s] Planck’s constant, after the scientist Max Planck. The energy of the light is proportional to the frequency (inversely proportional to the wavelength) ! The higher the frequency (lower wavelength) the higher the energy of the photon. 10 photons have an energy equal to ten times a single photon. Quantum theory describes experiments to astonishing precision , whereas the classical wave description canno Photoelectric Effect - Example Most metals have a work function on the order of several electron volts. Copper has a work function of 4.5 eV. Therefore, the cut-off frequency for light ejecting electrons from copper is: hf cutoff = 4.5 eV, or f cutoff = 4.5 x (1.6 x 10 -19 C) x (1 V) / 6.63 x 10 -34 J-sec = 1.09 x 10 15 Hz or cutoff = c/ f cutoff , or cutoff = (3 x 10 8 m/s) / (1.09 x 10 15 cycles/sec) = 276 nm (in the UV range) Any frequency lower than the cut-off (or any wavelength
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lecture18 101028 - Photons Photoelectric Effect - Example...

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