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

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

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 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