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PhotoElectric effect

Reading Recommendation: Pages 475-480. PhotoElectric effect. A phenomenon in which a metal is struck by incoming light, which causes electrons from the metal to be emitted from the metal’s surface. Examples where this can be observed:

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PhotoElectric effect

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  1. Reading Recommendation: Pages 475-480 PhotoElectric effect

  2. A phenomenon in which a metal is struck by incoming light, which causes electrons from the metal to be emitted from the metal’s surface. • Examples where this can be observed: • Light shining on a negatively-charged electroscope causes it to be discharged • Light shining on a neutral electroscope causes it to become charged (positively) Photoelectric Effect

  3. What is a photon? • It is a “packet” of light • “Quantized” light • Light acting as if it were a particle, not a wave, that is carrying its energy. • Wait…Quantized? What kind of word is that? • It means that there is a distinct, discrete amount of energy being carried by a “particle” of light, dependent on the qualities of the light. • Waves carry energy continuously, and in varying amounts/intensities depending on the position within the wave at which you look…continuous ≠ discrete Photons…and what Einstein thought of them

  4. Radiant light is emitted from objects that have a distinct amount of thermal energy (i.e. those objects that are at a measurable temperature emit radiant energy, the magnitude of which is dependent on the temperature) If the kinetic energy of the atoms is discrete And if the kinetic energy determines the temperature… Shouldn’t the radiant energy being emitted also be discrete? Einstein’s Logic…

  5. IF the radiant energy being emitted from the hotter object is NOT discrete, then there is no way to show that energy is conserved during the thermal energy transfer from hot object to cooler object/surroundings • However, it has been shown time and again that energy IS conserved. • So Einstein is probably right.  • (and earned the 1921 Nobel Prize in Physics as a result) And What if it’s not?

  6. Demo 1: https://www.youtube.com/watch?v=WO38qVDGgqw Demo 2: Phosphorescent strip (done in person in class ) Demo 3 (watch at home): https://www.youtube.com/watch?v=kcSYV8bJox8 Demos of this effect:

  7. Reminder: • An electronvolt (eV) is a measure of the amount of energy gained by an electron as it travels through a potential difference of exactly 1 V 1 eV = 1.6 x 10-19 J • We will be using electronvolts as our energy unit throughout this topic! Electronvolt

  8. Objects with measurable temperatures will radiate heat in the form of radiant energy Depending on the temperature, different wavelengths of light will be emitted Black Body Radiation

  9. Relates the wavelength of light with the temperature of the Black Body: For example…determine the surface temperature of an object that predominantly emits in the visible spectrum: Wien Displacement Law

  10. Relates the total energy being emitted by the blackbody to its temperature: Stefan-Boltzmann Law

  11. Combining the Wien law and the Stefan-Boltzmann law, it was shown, classically, that What happens as the wavelength gets shorter? Ultraviolet Catastrophe! Classical Theory result:

  12. Assumptions made to explain why the ultraviolet catastrophe doesn’t actually happen • Thermal Oscillators (the atoms emitting the radiation) have a specific, discrete amount of energy • Unlike blackbody radiation, the energy was not emitted in a broad, continuous spectrum • Energy was emitted in packets of energy, equivalent to the amount of energy contained by the atoms. Max Planck

  13. Planck’s hypothesis • Frequency of the emitted radiation is proportional to the energy the radiation carries away from the thermal oscillator. • Planck’s constant (an experimental value!) = 6.63 x 10-34 J·s = 4.14 x 10-15eV·s

  14. The term given to the quantity hf Planck’s hypothesis is used to explain quite a few phenomena not otherwise following a classical explanation. Quantum of energy

  15. Light carries a specific (discrete) amount of energy: Planck’s hypothesis connects the wave nature of light with the particle nature of photons (i.e. wave-particle duality) Helps explain the photoelectric effect, which cannot be explained classically. Quantum of light

  16. Calculate the energy of a photon of ultraviolet light, wavelength 3.00 x 10-7 m. Express your answer in both Joules and in electronvolts. #1

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