1.

A concave lens with unequal radii of curvature made of glass `(mu_(g)=1.5)` has a focal length of 40cm. If it is immersed in a liquid of refractive index `mu_(1)=2`, thenA. it behaves like a convex lens of 80cm focal lengthB. it behaves like a convex lens of 20cm focal lengthC. its focal length becomes 60cmD. nothing can be said

Answer» Correct Answer - a.
`-(1)/(40)=(1.5-1)((1)/(R_(1))-(1)/(R_(2)))`
`(1)/(R_(1))-(1)/(R_(2))=-(1)/(20)`
Now, `(1)/(f)=((1.5)/(2)-1)(-(1)/(20))`
`=-(0.5)/(2)(-(1)/(20))`
`=(1)/(80)` or `f=80cm`
It behaves like a convex lens of focal length 80 cm.


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