Two convex lenses of focal length `f_1` and `f_2` are mounted coaxially separated by a distance. If the power of the combination is zero, the distance between the lenses is

Two convex lenses of focal lengths f_(1) and f_2 are separated co-axially by a distance d. The power of the combination will be one if

A convex lens of focal length 25 cm and a concave lens of focal length 20 cm are mounted coaxially separated by a distance d cm . If the power of the combination is zero, d is equal to

Two thin convex combin lenses each of focal lenght 0.5 m are kept coaxially separated by a distance 0.5 m. Focal length combination is

Two thin convex lenses of focal lengths f_1 and f_2 are placed at a distance d between them. For the power of combination to be zero, the separation d should fee :

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

Two lenses of focal legths f_(1) and f_(2) are kept in contact coaxially. The resultant power of combination willl be

Two thin convex lenses of focal lengths 4 cm and 8 cm are separated by a distance of 4 cm in air. The combination will have the focal length

Two thin lenses of focal length f_1 and f_2 are in contact and coaxial.The power of the combination is