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

Concept: The photoelectric effect demonstrates the particle nature of light:

Emission of electrons occurs instantly when light of sufficient frequency falls on a metal
No measurable time delay

Step 1: Analyze Assertion (A). Photoelectric emission occurs immediately once light of frequency above threshold strikes the surface. Hence, it is considered a spontaneous (instantaneous) phenomenon. Assertion is true.
Step 2: Analyze Reason (R). According to classical wave theory:

Energy is delivered continuously
Electron should accumulate energy gradually
This would cause a measurable time delay
But experiments show no delay. Thus, the reason correctly describes the classical prediction.
Step 3: Link between A and R. The absence of time lag (spontaneity) contradicts classical wave theory and supports photon theory. Hence, the reason explains why the effect is spontaneous. Conclusion: Both Assertion and Reason are true, and Reason correctly explains Assertion.