Question:
Assertion (A) : When a convex lens made of glass is immersed in water, its converging power increases.
Reason (R) : The focal length of a lens depends only on the radii of curvature of its two faces.
Assertion (A) : When a convex lens made of glass is immersed in water, its converging power increases.
Reason (R) : The focal length of a lens depends only on the radii of curvature of its two faces.
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When a convex lens is immersed in water, its power decreases because the refractive index contrast between lens and surrounding medium decreases.
Updated On: Jul 2, 2026
- Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A).
- Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A).
- Assertion (A) is true, but Reason (R) is false.
- Both Assertion (A) and Reason (R) are false.
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The Correct Option is D
Solution and Explanation
Concept:
The focal length of a lens in a medium is given by the lens maker's formula
\[
\frac{1}{f}
=
\left(
\frac{n_{\text{lens}}}{n_{\text{medium}}}
-1
\right)
\left(
\frac{1}{R_1}
-
\frac{1}{R_2}
\right).
\]
The focal length depends on both refractive indices and radii of curvature.
Step 1: Examine the Assertion. When a glass lens is immersed in water, \[ \frac{n_{\text{lens}}}{n_{\text{medium}}} \] decreases because water has a refractive index greater than air. Therefore, \[ \frac{1}{f} \] decreases and the focal length increases. Since power \[ P=\frac{1}{f}, \] the power decreases. Hence, the converging power does not increase. The Assertion is false.
Step 2: Examine the Reason. The focal length does not depend only on radii of curvature. It also depends on the refractive index of the lens material and the surrounding medium. Hence, the Reason is false.
Step 3: Final conclusion. Both Assertion and Reason are false. \[ \boxed{\text{(D)}} \]
Step 1: Examine the Assertion. When a glass lens is immersed in water, \[ \frac{n_{\text{lens}}}{n_{\text{medium}}} \] decreases because water has a refractive index greater than air. Therefore, \[ \frac{1}{f} \] decreases and the focal length increases. Since power \[ P=\frac{1}{f}, \] the power decreases. Hence, the converging power does not increase. The Assertion is false.
Step 2: Examine the Reason. The focal length does not depend only on radii of curvature. It also depends on the refractive index of the lens material and the surrounding medium. Hence, the Reason is false.
Step 3: Final conclusion. Both Assertion and Reason are false. \[ \boxed{\text{(D)}} \]
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