JEE Physics 2025 Question with Solution

Given:

• Assertion (A): Emission of electrons in photoelectric effect can be suppressed by applying a sufficiently negative electric potential to the photoemissive substance.
• Reason (R): The negative electric potential required to stop photoelectrons varies linearly with the frequency of incident radiation.

Find: Which option correctly describes the truth of (A) and (R) and whether (R) explains (A).

In the photoelectric effect, a sufficiently negative retarding potential can prevent the emitted electrons from reaching the anode. Therefore, Assertion (A) is true.

The stopping potential is related to incident frequency by

so $V_0$ varies linearly with $\nu$. Therefore, Reason (R) is also true.

However, (R) does not explain why a negative potential suppresses photoelectron emission current. It only states how stopping potential depends on frequency. The suppression occurs because the retarding field opposes the motion of emitted electrons.

Therefore, both (A) and (R) are true, but (R) is not the correct explanation of (A). The correct option is A.

Given:

• Light falling on a photoemissive surface causes emission of electrons.
• A negative potential may be applied to oppose these emitted electrons.

Find: The correct assertion–reason relation.

Step 1: Analyze Assertion (A). A sufficiently negative potential acts as a retarding potential. When this opposing potential is large enough, even the most energetic photoelectrons cannot move against it to reach the collector, so the photoelectric current becomes zero. Hence, (A) is true.

Step 2: Analyze Reason (R). From Einstein's photoelectric equation,

where $e$ is electron charge, $h$ is Planck's constant, $\nu$ is frequency, and $\phi$ is work function. This is a linear relation between $V_0$ and $\nu$. Hence, (R) is true.

Step 3: Check whether (R) explains (A). The assertion is about the role of a negative potential in stopping emitted electrons. The reason is about the dependence of stopping potential on frequency. Although related to the same phenomenon, the linear dependence on frequency does not explain the mechanism by which negative potential suppresses emission current.

So, both statements are true, but the reason is not the correct explanation of the assertion. Hence, the correct option is A.