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# Form 4 Physics Photoelectric Effect Questions and Answers

(a) It is observed that when ultra-violet radiation is directed onto a clean zinc plate connected to the cap of a negatively charged leaf electroscope, the leaf falls.
(i) Explain this observation.
(ii) State why this observation does
not occur if the electroscope is positively charged.
(iii) Explain why the leaf of the electroscope does not fall when infra-red radiation is directed onto the zinc plate.
(b) State the effect on the elctrons emitted by the photoelectric effect when:
(i) The intensity of incident radiation is increased.
(ii) The frequency of the incident radiation is increased.
(c) The maximum wavelength required to cause photoelectric emission on a metal surface is 8.0 x 10^-7m. The metal surface is irradiated with light of frequency 8.5 x10^14Hz. Determine:
(i) The threshold frequency.
(ii) The work function of the metal in electron volts.

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