A cesium photocell, with a steady potential difference of 60V across it, is illuminated by a small bright light placed 1 m away. When the same light is placed 2m away, the number of electrons crossing the photocell:
Correct answer: C. are one-quarter as numerous
- A. each carries one-quarter of their previous momentum
- B. each carries one-quarter of their previous energy
- C. are one-quarter as numerous
- D. are half as numerous
Explanation
The intensity of light is inversely proportional to the square of the distance between the light source and the photocell. When the light source is moved from 1 m to 2 m away, the intensity of light is reduced to 1/4 of the original intensity. This means that the number of electrons emitted by the photocell is also reduced to 1/4 of the original number.
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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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