Question

What will be power when we emerge lense in water

Answer 1

The speed of light is not constant for all mediums.The speed varies based on the optical density of the medium.
Optical density can be somewhat thought as the density of the medium but not necessarily and optical density is inversely proportional to speed of light in medium.
Now,you can guess that the optical density of glass is more than that of air.Experomentally it has been found that the speed of light in air is roughly 3*10^8 and in glass is 2*10^8.