Two lenses have focal lengths `f_(1)` and `f_(2)` and their dispersive powers are `omega_(1)` and `omega_(2)` respectively. They will together form an achromatic combination if

Focal length of two lens are f and f^(') ans dispersive powers are omega_(0) and 2omega_(0) .To form achromatic combination from these- (1) f^(')=2f ,(2) f^(')=-2f (3) f^(')=-f//2

Focal length of two lens are f and f^(') and dispersive powers are omega_(0) and 2omega_(0) .To form achromatic combination from these- (1) f^(')=2f ,(2) f^(')=-2f (3) f^(')=-f//2

Focal lengths of two lenses are f and f' and dispersive powers of their materials are omega and omega' . To form achromatic combination from these, which relation is correct ?

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