A thin equiconvex lens is made of a glass of refractive index 1.5 and its focal length is 0.2 m. If it acts as a concave lens of 0.5 m focal length when dipped in a liquid, the refractive index of the liquid is:
178
158
138
98
A
98
B
138
C
178
D
158
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Solution
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Given,
μ1=1.5f1=0.2mf2=(−0.5)μ2=?
Using lens makers formula and taking ratios, f1f2=(μ2−1)(μ1−1)
⇒0.2(−0.5)=(1.5μe−1)(1.5−1)
⇒0.2×0.50.5=(1−1.5μe)
⇒1.5μe=0.8
⇒μe=158
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