When a thin convex lens is put in contact with a thin concave lens of the same focal length (f), then the resultant combination has focal length equal to

A thin convex lens is put in contact with a thin concave lens of equal focal length. The effective power and focal length of the lens combination respectively are

A convex lens of focal length f is put in contact with a concave lens of the same focal length. The equivalent focal length of the combination is :

A convex lens of focal length 40cm is in contact with a concave lens of focal length 25cm. The power of the combination is

A convex lens of focal length cc"f" is put in contact with a concave lens of the same focal length. The equivalent focal length of the combination is :

A convex lens of focal length 40 cm is in contact with a concave lens of focal length 25 cm. The power of the combination is

A convex lens of focal length 40 cm is in contact with a concave lens of focal length 25 cm. The power of the combination is

A convex lens of focal length 'f' is placed in contact with a concave lens of the same focal length. The equivalent focal length of the combination is