Correct Option: Assertion (A) is false and Reason (R) is false.

Step 1 (Formula / Governing Law & Given Data): [0.5 Mark]

The focal length of a lens in a medium is governed by the Lens Maker's Formula:

Where:

• $f$ = focal length of the lens
• $\mu_l$ = refractive index of the lens material
• $\mu_m$ = refractive index of the surrounding medium
• $R_1, R_2$ = radii of curvature of the lens surfaces

Step 2 (Step-by-Step Mathematical Working): [0.25 Mark]

When the lens is immersed in water ($\mu_w \approx 1.33$) instead of air ($\mu_a \approx 1.0$), the ratio $\frac{\mu_l}{\mu_m}$ decreases. Since $\mu_l > \mu_m$, the term $\left( \frac{\mu_l}{\mu_m} - 1 \right)$ decreases. Consequently, the value of $\frac{1}{f}$ decreases, which implies that the focal length $f$ increases. Since Power $P = \frac{1}{f}$, the converging power of the lens decreases. Thus, Assertion (A) is false.

Regarding Reason (R), the Lens Maker's Formula explicitly shows that focal length depends on both the radii of curvature and the refractive indices of the lens and the surrounding medium. Thus, Reason (R) is false.

Step 3 (Final Concluded Answer): [0.25 Mark]

Both Assertion (A) and Reason (R) are incorrect. The converging power decreases because the relative refractive index between the lens and the medium decreases, and the focal length depends on the medium as well as the geometry of the lens.

💡 EL10 Concept Insight: Think of a lens like a prism that bends light; it bends light better when there's a big difference between the lens material and the outside air. When you put the lens in water, the "difference" becomes smaller, so the lens can't bend the light as sharply, making its power weaker.