JEE Mains · Physics · STD 12 - 9. Ray optics and optical instruments
A biconvex lens of refractive index \(1.5\) has a focal length of \(20 \mathrm{~cm}\) in air. Its focal length when immersed in a liquid of refractive index \(1.6\) will be:
- A \(-16 \mathrm{~cm}\)
- B \(-160 \mathrm{~cm}\)
- C \(+160 \mathrm{~cm}\)
- D \(+16 \mathrm{~cm}\)
Answer & Solution
Correct Answer
(B) \(-160 \mathrm{~cm}\)
Step-by-step Solution
Detailed explanation
\( \mu_1=1.5 \) \( \mu_{\mathrm{m}}=1.6 \) \( \mathrm{f}_{\mathrm{a}}=20 \mathrm{~cm}\) \( \text { As } \frac{f_m}{f_a}=\frac{\left(\mu_1-1\right) \mu_m}{\left(\mu_1-\mu_m\right)} \) \( \frac{f_m}{20}=\frac{(1.5-1) 1.6}{(1.5-1.6)} \) \( f_m=-160 \mathrm{~cm}\)
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