(i) If the lens is placed in water instead of air, how does its focal length change ?
(ii) Which lens, thick or thin has greater focal length ?
(i) No. Focal length is the distance between lens plane and and focal plane. Focal does not depend on external parameter.
Since the relative refractive index of glass with respect to water is less than that of glass with respect to air, the bending of light at the interface of glass water is lesser and hence a parallel beam of light falling on the lens is bent less, to meet at point on the principal axis and hence the lens has more focal length in water than in air. the focal length increases on placing inside the water.
(ii) Thin lens