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A biconvex lens of refractive index 1.5 has a focal length of 20 cm in air. Its focal length when immersed in a liquid of refractive index 1.6 will be:
(4) -160 cm
1f= ( l m-1 ) ( 1R1- 1R2 ) (lens maker's formula)
fmfa= l-1 l m-1= 0.5 1.51.6-1= 0.5- 116=-8
fm=-8 20=-160 cm
l< m, the biconvex lens behaves as a diverging lens.
Answer: (4)
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