CBSE 2025, Set 3

Question

**Assertion (A):** The minimum negative potential applied to the anode in a photoelectric experiment at which photoelectric current becomes zero, is called cut-off voltage.

**Reason (R):** The threshold frequency for a metal is the minimum frequency of incident radiation below which emission of photoelectrons does not take place. 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.

Edvaya · Accepted Answer

The correct answer is Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A).. **Step 1: Check Assertion (A)**
The definition given for cut-off voltage (or stopping potential) is standard and correct. It is the retarding potential that stops even the most energetic photoelectrons. (Assertion is **True**).

**Step 2: Check Reason (R)**
The definition for threshold frequency is also standard and correct. Below this frequency, photons lack sufficient energy to overcome the work function of the metal. (Reason is **True**).

**Step 3: Check Explanation**
While both statements define key terms in the photoelectric effect, the concept of threshold frequency (Reason) does not explain the physical meaning of stopping potential (Assertion). Stopping potential relates to the maximum kinetic energy of emitted electrons, not directly the existence of a threshold.

Thus, both A and R are true, but R is NOT the correct explanation of A.

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Physics > Modern Physics > Dual Nature Of Radiation And Matter

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