A thin equi-convex lens, each surface of which has a radius of curvature of 6cm, ismade in a plastics material of refractive index 1.498, and is placed in a water tank,50cm from an object of height 4mm. A screen is placed in the water tank behindthe lens and is adjusted until a sharp image is formed on the screen. How far is thescreen from the lens and what is the size of the image? (Assume the refractiveindex of the water to be 1.334.)

How would I go about answering this question? I've tried to find 'F' in so many different ways but nothing makes sense. Please help!!!