Appeared in: 2026 (3 sets)
Assertion (A) : On increasing the intensity of incident light of frequency $\nu (> \nu_0)$ on a photosensitive surface, the photocurrent increases.
Reason (R) : The stopping potential for a photosensitive surface increases with increase of frequency $\nu (> \nu_0)$ of incident light.
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Official Evaluation Steps & EL10 Insight
Question Analysis: Assertion (A) and Reason (R) type question.
Step 1 (Governing Principles): [0.5 Mark]
- Assertion (A): The photocurrent is directly proportional to the number of photoelectrons emitted per second, which depends on the intensity of incident light (number of photons per unit area per unit time).
- Reason (R): According to Einstein’s photoelectric equation: $$K_{max} = h\nu - \phi_0 = eV_s$$ where $K_{max}$ is maximum kinetic energy, $\nu$ is frequency, $\phi_0$ is work function, and $V_s$ is stopping potential.
Step 2 (Logical Deduction): [0.25 Mark]
- For Assertion (A): Increasing intensity increases the number of incident photons, thereby increasing the number of emitted photoelectrons. Thus, the photocurrent increases. (Assertion is True)
- For Reason (R): From the equation $V_s = \frac{h\nu}{e} - \frac{\phi_0}{e}$, it is evident that $V_s$ is a linear function of frequency $\nu$. As $\nu$ increases, $V_s$ increases. (Reason is True)
Step 3 (Final Concluded Answer): [0.25 Mark]
Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of Assertion (A). The increase in photocurrent (A) is due to intensity, whereas the increase in stopping potential (R) is due to frequency; these are two independent phenomena of the photoelectric effect.
💡 EL10 Concept Insight:
Think of light like a rain of balls: Intensity is how many balls you throw (more balls = more splashes/current), while Frequency is how hard you throw each ball (harder throw = more energy/stopping potential). They are two different ways to change the outcome!