JEE Physics 2025 Question with Solution

Given: Assertion $A$ says that in photoelectric effect, increasing the intensity of incident light increases the stopping potential. Reason $R$ says that increasing intensity increases the rate of photoelectrons emitted, provided the frequency of incident light is greater than the threshold frequency.

Find: Which option correctly evaluates Assertion $A$ and Reason $R$.

The governing relation is

$$eV_0 = K_{\text{max}} = h\nu - \phi$$

Here, the stopping potential $V_0$ depends on the frequency $\nu$ of incident light and the work function $\phi$, not on the intensity.

So, if the frequency remains unchanged, increasing the intensity does not increase the stopping potential. Therefore, Assertion $A$ is false.

When intensity increases, the number of incident photons per second increases. If $h\nu > \phi$, more electrons are emitted per unit time. Thus, the rate of photoelectron emission increases. Therefore, Reason $R$ is true.

Hence, Assertion $A$ is false but Reason $R$ is true. The correct option is B.