Glass lenses behave in the way it is expected to behave when the medium they are in is a vacuum or air. But if the medium is also glass it obviously doesn’t work as a lens. If the density of the medium is higher than that of glass the lens works in the opposite way. The power of a lens depends on the medium it is in. So if we consider a lens that’s sunk in water it’s not as powerful as it was when the medium was air. The power of a lens= 1/focal length When the power decrease the focal lens should increase. But as “2F=C” the c should also increase. Yet c is defined as “The radius of the glass sphere that was used to make the lens” If the definition is correct C cannot be changed. Either the definition is wrong or the principals we use to specify the path of rays of light are only valid when the medium is air or a vacuum.