Two thin lenses of power `+ 3 D and - 1 D` are held contact with each other. 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 - 15 D and + 5 D are in contact with each other. The focal length of the combination is

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

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

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

Two lenses of power + 15 D and - 5 D are in contact with each other. What is the focal length of the combination ? What would be the position of image formed by the combination for an object at 30 cm ?

Two lens of power - 12D and + 7D are in contact with each other, the focal length of combination is :

If two lenses of power 2D and 3D are kept in contact with each other, then focal length of the combination will be .......... cm.

Two thin lenses having optical powers of -10 D and + 6 D are placed in contact with each other. The focal length of the combination is :