A convex lens made up of glass of refractive index 1.5 is dipped, in turn, in (i) a medium of refractive index 1.65, (ii) a medium of refractive index 1.33.
(a) Will it behave as a converging or a diverging lens in the two cases?
(b) How will its focal length change in the two media?Given, refractive index of glass, ng = 1.5
Therefore, focal length of lens in liquid, 
a) i) ng = 1.5, nl
So fi and fa are of opposite sign.
Therefore, convex in liquid 1 behaves as a diverging lens.
ii) Refractive index of medium, nl = 1.33
Therefore, fl and fa are of the same sign.
So, convex lens in liquid l behaves a s a convergent lens.
b)
i) Focal length, f1 is, 
Focal length becomes negative and it’s magnitude increases.
ii) Focal length f2 is given by, 
Focal length increases in magnitude and is positive.
