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The photoelectric effect

The photoelectric effect. To be a wave or a particle? That is the question. Quick overview:. X-ray machine hits target with electrons and EM radiation flies out In the photoelectric effect, you hit target with EM radiation and electrons fly out!

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The photoelectric effect

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  1. The photoelectric effect To be a wave or a particle? That is the question.

  2. Quick overview: • X-ray machine hits target with electrons and EM radiation flies out • In the photoelectric effect, you hit target with EM radiation and electrons fly out! • The electrons ejected from the target are called “photoelectrons”

  3. The setup • An adjustable voltage is applied. Voltage can be forward or reverse biased (which slows down the electrons) • Photoelectrons return to cathode through an ammeter which records the current

  4. Some experimental results: • If photoelectrons get ejected when you shine monochromatic light on the target, the current increases when you increase the intensity (brighter light = more photoelectrons) • BUT…above a “cutoff wavelength” no photoelectrons get ejected no matter how great the intensity of the incident radiation • AND…for wavelengths below the cutoff, decreasing the radiation to very low intensities does not completely eliminate the production of photo electrons

  5. High intensity (bright) Stopping potential Low intensity (dim) Current vs. Bias voltage Reverse bias Forward bias

  6. Stopping voltage vs. Frequency (c/l) eV(stopping) frequency

  7. Interpretation • Slope is same for all targets • y intercept is different for different target materials. • eDV=hf - fmetal • f is the “work function” of the metal…the mininum amount of energy required to remove an electron. • h=6.63 x 10-34 is Planck’s Constant!

  8. Interpretation (continued) • Planck’s EM “quanta” turn out to be real after all! (?) • Light comes in energy packets equal to hf • Each packet acts more like a particle than a wave • These light “particles” are called photons • Rather than continuously absorbing wave radiation the target is being bombarded by photons like tiny billiard balls!

  9. Concluding statements • Einstein figured out the photoelectric effect in 1905 (the same year he developed the theory of special relativity and explained Brownian Motion). This is what he got the Nobel prize for. • I like to think of the the photoelectric equation in terms of conservation of energy light energy ejecting electron & KE KE  eDV hf = f + KE = f + eDV

  10. Parting question... So is light a particle? Or is light a wave?

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