Question

A concave lens of glass, refractive index 1.5 has both surfaces of same radius of curvature R. On immersion in a medium of refractive index 1.75, it will behave as a
A. Convergent lens of focal length `3.5R`
B. Convergent lens of focal length `3.0R`
C. Divergent lens of focal length `3.5R`
D. Divergent lens of focal length `3.0 R`

Answer 1

Correct Answer - A
`(f_(I))/(f_(a))=((._(a)mu_(g)-1))/((._(I)mu_(g)-1))=(1.5-1)/((1.5)/(1.75)-1)=(1.75xx0.50)/(0.25)=-3.5`
`:. f_(I)=-3.5f_(a)impliesf_(I)=+3.5R(because f_(a)=R)`
Hence on immersing the lens in the liquid, it behaves as a converging lens of focal length `3.5R`.