In an experiment, a tungsten cathode with a threshold wavelength of 2300 Å is irradiated by ultraviolet light of wavelength 1800 Å. The maximum energy of emitted photoelectrons will be:
Correct answer: B. 1.5 eV
- A. 1.2 eV
- B. 1.5 eV
- C. 1.6 eV
- D. 1.8 eV
Explanation
Threshold energy Φ = hc/λ₀ = 1240/2300 ≈ 5.39 eV. Photon energy E = hc/λ = 1240/1800 ≈ 6.89 eV. Maximum KE = E - Φ = 6.89 - 5.39 ≈ 1.5 eV.
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About Dawn of Modern Physics
Classical physics cannot explain blackbody radiation and related observations, leading to Planck’s quantum theory, in which energy is emitted or absorbed in discrete packets. Photons provide the particle model of light, with energy and momentum linked to frequency and wavelength, including the photoelectric effect and its threshold frequency.
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