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` ?

Derive the lens formula, (1)/(2)=(1)/(v)-(1)/(u) for a concave lens, using the necessary ray diagram. Two lenses of powers 10 D and -5 D are placed in contact. (i) Calculate the power of the new lens. (ii) Where should an object be held from the lens, so as to obtain a virtual image of magnification 2?

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

Two lenses of power -15D and +5 D are in contact with each other. The focal length of the combination is

Two thin lenses of power + 6 D and - 2 D are in contact. What is the focal length of the combination ?

Two thin lenses of power +5 D and -2.5 D are in contact. What is the focal length of the combination ?

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