Question

How the focal length of a convex lens of glass affected if
i) lens is placed in water
ii) lens is placed in a liquid of larger refractive index than glass
iii) lens becomes thicker at middle

Answer 1

$\frac{1}{f}=(\frac{{\mu }_2}{{\mu }_1}-1)(\frac{1}{R_1}-\frac{1}{R_2})$
When placed in water, $\frac{{\mu }_2}{{\mu }_1}$ will decrease, so focal length will increase.
When placed in greater ${\mu }_1$, the focal length will become negative (so it will behave as a concave lens).