Question

Assertion : Photoelectric effect is a spontaneous phenomenon. Reason (R): According to the wave picture of radiation, an electron would take hours/days to absorb sufficient energy to overcome the work function and come out from a metal surface.

• A. Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A).
• B. Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A).
• C. Assertion (A) is true, but Reason (R) is false.
• D. Both Assertion (A) and Reason (R) are false.

Hint:

Key photoelectric observations:

Instant emission (no time lag)
Threshold frequency exists
Supports photon (quantum) theory of light

Answer 1

The Correct Option is A

The given assertion and reason require us to analyze the photoelectric effect and its explanation through the wave theory of light.

• Assertion (A): Photoelectric effect is a spontaneous phenomenon.
  • The photoelectric effect refers to the emission of electrons from a metal surface when it is exposed to light of a certain frequency or higher.
  • This effect is indeed spontaneous because electrons are emitted almost instantaneously as light of sufficient energy strikes the metal surface.
• Reason (R): According to the wave picture of radiation, an electron would take hours/days to absorb sufficient energy to overcome the work function and come out from a metal surface.
  • The wave theory of light suggested that light energy is spread over the wavefront. Thus, it would require a prolonged exposure to accumulate enough energy to eject an electron, which contradicts the instantaneous nature of the observed photoelectric effect.
  • This discrepancy between wave theory and experimental observations led to the proposal of the quantum theory of light, where light comprises particles (photons) with energy directly proportional to their frequency.

Conclusion: Both Assertion (A) and Reason (R) are true. The assertion correctly states the spontaneous nature of the photoelectric effect, and the reason explains why the wave theory of light could not account for this phenomenon, leading to the development of quantum theory. Thus, Reason (R) is indeed the correct explanation of the Assertion (A).