power 4d6. A convex lens of power +6 D is placed in contact with a concave lens of power –4D. What will be the nature and focal length of this combination ? (a) convex, 100 cm (b) convex, 50 cmAnswer:the power of a convex lens is always positive. as you can see here the power is +4D, therefore the nature of the lens is that it is a convex lens. we know, P = 1f. also