A cesium photocell, with a steady potential difference of 60V across it, is illuminated by a bright point source of light 50cm away. When the same light is placed 1m away, the photoelectrons emitted from the cell:
Correct answer: A. Are one quarter as numerous
- A. Are one quarter as numerous
- B. Are half as numerous
- C. Each carry one quarter of their previous momentum
- D. Each carry one quarter of their previous energy
Explanation
Intensity decreases with distance squared, I' = I/(2)² = I/4. Photoelectron number is proportional to intensity, reducing to one-quarter.
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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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