Two thin lines of powers -4D and 2D are placed in contact coaxially. Find the focal length of the combination.

Two lenses of powers -1.5 D and +2.75D are kept in contact. Find the focal length of the combination.

Two lenses of power + 10 D and - 5 D are placed in contact, (i) Calculate the focal length of the combination (ii) where should an object be held from the combination so as to obtain a virtual image of magnification 2 ?

Two lenses of power + 10 D and - 5 D are placed in contact, (i) Calculate the focal length of the combination (ii) where should an object be held from the combination so as to obtain a virtual image of magnification 2 ?

Two thin lenses of power 2D and -4D are kept in contact. What is the focal length of the combination in cm? (write the answer only)

Two lenses of power 15D and -3D are placed in contact. The focal length of the combination is a

Two lenses of power 15D and -3D are placed in contact. The focal length of the combination is a