Question

Assertion (A) : 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:

The instantaneous nature of photoelectric emission is one of the strongest evidences for the particle nature of light and cannot be explained by classical wave theory.

Answer 1

The Correct Option is B

Concept: One of the most important observations of the photoelectric effect is the absence of any measurable time lag between the incidence of light and the emission of electrons. As soon as light of frequency greater than the threshold frequency falls on a metal surface, electrons are emitted almost instantaneously. This behaviour could not be explained by classical wave theory but is naturally explained by Einstein's photon theory.

Step 1: Examine the Assertion. Photoelectric emission occurs immediately after suitable radiation falls on the metal surface. There is practically no time delay. Hence, photoelectric emission is considered a spontaneous or instantaneous phenomenon. Therefore, the Assertion is true.

Step 2: Examine the Reason. Classical wave theory predicts that energy is absorbed continuously. Therefore, at low intensities, an electron would require a very long time to accumulate enough energy to escape from the metal surface. This prediction contradicts experimental observations. Thus, the Reason is true.

Step 3: Check whether the Reason explains the Assertion. The reason merely states a prediction of classical wave theory. The actual explanation of the spontaneous nature of photoelectric effect comes from Einstein's photon theory, according to which an electron absorbs energy in a single photon-electron interaction. Hence, although the Reason is true, it is not the correct explanation of the Assertion.

Step 4: Final conclusion. \[ \boxed{\text{Assertion is true}} \] \[ \boxed{\text{Reason is true}} \] but \[ \boxed{\text{Reason is not the correct explanation of the Assertion}}. \] Therefore, \[ \boxed{\text{(B)}}. \]