JEE Main 2024 Physics question
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 :
Answer: (C)
Solution
In air 1/f = (1.5 − 1)(2/R) gives R = 20 cm. In the liquid 1/f = (1.5/1.6 − 1)(2/20) = −1/160, so f = −160 cm.