Two lenses of power `+12` and `-2` dioptres are placed in contact. The combined focal length of the combination will be

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

Two thin lenses of power + 3 D and - 1 D are held in contact with each other. Focal length of the combination is :

Two lenses of power -1.75D and +2.25 D respectively, are placed in contact. Calculate the focal length of the combination.

Two coaxial lenses of power + 4D and - 2D are placed in contact. The focal length of combination is

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