JEE Mains · Physics · STD 12 - 9. Ray optics and optical instruments

A thin plano convex lens made of glass of refractive index 1.5 is immersed in a liquid of refractive index 1.2. When the plane side of the lens is silver coated for complete reflection, the lens immersed in the liquid behaves like a concave mirror of focal length 0.2 m . The radius of curvature of the curved surface of the lens is _______.

A 0.20 m
B 0.25 m
C 0.15 m
D 0.10 m

Verified Solution

Answer & Solution

Correct Answer

(D) 0.10 m

Step-by-step Solution

Detailed explanation

\begin{aligned} & \frac{1.5}{\mathrm{v}}=\frac{1.5-1.2}{\mathrm{R}} \\ & \mathrm{v}=\frac{1.5 \mathrm{R}}{0.3}=5 \mathrm{R} \\ & \frac{1.2}{\mathrm{f}}-\frac{1.5}{5 \mathrm{R}}=\frac{1.2-1.5}{-\mathrm{R}} \\ & \frac{1.2}{\mathrm{f}}=\frac{0.3}{\mathrm{R}} \times 2 \Rightarrow…

Apply the relevant identity from the chapter and substitute the values established in the previous step, keeping the original constraint in view.

Use the simplified expression to isolate the unknown, then plug the result back into the question setup to confirm the working is internally consistent.

Cross-check against a boundary or limiting case. Both the dimensional balance and the qualitative behaviour at the extreme should agree with the candidate answer.

Combine the steps above to identify the correct option. The remaining choices each fail at least one of the consistency, dimensional, or boundary checks.

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List - \(I\) \((Number)\)	List - \(II\) \((Significant figure)\)
\((A)\) \(1001\)	\((I)\) \(3\)
\((B)\) \(010.1\)	\((II)\) \(4\)
\((C)\) \(100.100\)	\((III)\) \(5\)
\((D)\) \(0.0010010\)	\((IV)\) \(6\)