Derive the relation : `(1)/(F) =(1)/(f_(1)) +(1)/(f_(2))`,where `f_(1)` and `f_(2)` are focal length of two thin lenses and F is the focal length of the combination in contact.

Two thin lenses of focal lengths f_(1) and f_(2) are in contact. The focal length of this combination is

Deduce the equivalent focal length of two convex lenses of focal lengths F_(1) and F_(2) when placed in contact.

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

Two thin lenses of focal length f_(1) and f_(2) are in contact and coaxial. The power of the combination is

Power (P) of a lens is given by reciprocal of focal length (f) of the lens. i.e. P = 1//f . When f is in metre, P is in dioptre. For a convex lens, power is positive and for a concave lens, power is negative. When a number of thin lenses of powers p_(1), p_(2), p_(3).... are held in contact with one another, the power of the combination is given by algebraic sum of the powers of all the lenses i.e., P = p_(1) + p_(2) + p_(3) + ........ Answer the following questions : Two thin lenses are in contact and the focal length of the combination is 80 cm . If the focal length of one lens is 20 cm , the focal length of the other would be

Two thin convex lenses of focal lengths f_(1) and f_(2) are in contact and coaxial. The power of the combination is