In an experiment, a tungsten cathode that has a threshold wavelength of 2300 Å is irradiated by ultraviolet light of wavelength 1800 Å. The maximum energy of emitted photo-electrons 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
The light used (1800 Å) is more energetic than the minimum required (2300 Å).The difference in their energies is moderate, so the emitted electrons will have a moderate, not too small and not too large kinetic energy.1.5 eV fits exactly with the expected amount of extra energy.
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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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