A certain piece of equipment contains a lens, made of glass [n = 1.60], that has a focal length of 40 cm. In order for the equipment to operate correctly this focal length must be strictly adhered to.
Unfortunately that portion of the apparatus containing the lens - which was originally designed to operate in air - must now be submerged in a rectangular vessel of water to cool the surrounding mechanisms. In order for the apparatus to operate properly while under water, a second lens, of the same type of glass, must be placed in the water in close combination with the original lens to bring the lens system back into focus.