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Photoelectric Effect - Hallwachs’ and Lenard’s Observations

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Estimated time: 3 minutes
CBSE: Class 12

Key Points: Photoelectric Effect — Hallwachs' and Lenard's Observations

Hallwachs' Observation:

Hallwachs confirmed that UV light incident on a negatively charged zinc plate caused it to lose charge (emit electrons).

Lenard's Observations:

  • Lenard measured electron kinetic energy vs light frequency.
  • Found: Maximum KE of emitted electrons is directly proportional to the frequency of incident light.
  • Changing the intensity of light had no effect on kinetic energy — only on the number of electrons emitted.
  • Below a certain threshold frequency (ν₀), no electrons are emitted regardless of intensity.
  • The photocurrent was directly proportional to the intensity of the incident light.
  • Setup: cathode illuminated with light → electrons travel through vacuum → reach anode → current measured via ammeter.

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