Question

if a lense is placed in water why does it increase its focal length

Answer 1

its because when it is placed in water the object seems to be biggger than they normally apper

Answer 2

Lenses work by REFRACTION, not reflection, and the angle of refraction is based on the difference between the speed of light in the lens compared to the surrounding medium. When immersed in water, the difference between light's speed in the lens and the surrounding medium is less than the difference when it's in air, and the focal length of a lens would be greater under water.