Two concave lenses `L_1 and L_2` are kept in contact with each other. If the space between the two lenses is filled with a material of smaller refractive index, the magnitude of the focal length of the combination

Two identical equiconvex lenses of refractive index 3/2 and focal length 16cm are kept in contact. The space between the lenses is filled with water of refractive indext 4/3 . What is the focal length of the combination?

Two identical equiconvex lenses of refractive index 3//2 and focal length 16 cm are kept in contact. The space between the lenses is filled with water of refractive index 4//3 . What is the focal length of the combination ?

Two plano concave lenses of glass of refractive index 1.5 have radii of curvature 20 cm and 30 cm respectively. They are placed in contact with the curved surface towards each other and the space between them is filled with a liquid of refractive index 5/2. The focal length of the combination is (in cm)

Two convex lenses with power 2 dioptre are kept in contact with each other. The focal length of the combined lens system is