Question:
A thin lens of glass of refractive index 1.5 has focal length 24 cm in air. It is now immersed in a liquid of refractive index \( \frac{9}{8} \). Its new focal length is:
A thin lens of glass of refractive index 1.5 has focal length 24 cm in air. It is now immersed in a liquid of refractive index \( \frac{9}{8} \). Its new focal length is:
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When a lens is immersed in a medium with a different refractive index, its focal length changes according to the relative refractive indices of the lens and the medium.
Updated On: Feb 9, 2026
- 72 cm
- 54 cm
- 36 cm
- 18 cm
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The Correct Option is C
Solution and Explanation
Step 1: Formula for Focal Length.
The focal length \( f \) of a lens is given by: \[ \frac{1}{f} = \left( \frac{\mu - 1}{R} \right) \] where \( \mu \) is the refractive index of the lens material and \( R \) is the radius of curvature. In air, the refractive index of the lens is 1.5, and in the liquid, the effective refractive index is \( \frac{9}{8} \). The new focal length can be calculated by adjusting the refractive index, yielding a new value of 36 cm.
Step 2: Final Answer.
Thus, the new focal length of the lens is 36 cm.
The focal length \( f \) of a lens is given by: \[ \frac{1}{f} = \left( \frac{\mu - 1}{R} \right) \] where \( \mu \) is the refractive index of the lens material and \( R \) is the radius of curvature. In air, the refractive index of the lens is 1.5, and in the liquid, the effective refractive index is \( \frac{9}{8} \). The new focal length can be calculated by adjusting the refractive index, yielding a new value of 36 cm.
Step 2: Final Answer.
Thus, the new focal length of the lens is 36 cm.
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