Two identical thin planoconvex glass lenses (refractive index `1.5)` each having radius of curvature of `20cm` are placed with their convex surfaces in contact at the centre. The intervening space is filled with oil of refractive index `1.7`. The focal length of the combination is

Two identical thin plano-convex glass lenses (refractive index 1.5) each having radius of curvature of 20 cm are placed with their convex surface in contact at the centre. The intervening space is filled with oil of refractive index 1.7. The focal length of the combination is :

Two identical thin plano-convex glass lensses (refractive index 1.5) each having radius of curvature of 20 cm are jplaced with their convex suffaces in contact at the cenjtre. The intervening space is filled with oil of refractive index 1.7 The focal length of the combination is

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 plano concave lenses of glass of refractive index 1.5 have radii of currature 20cm and 30cm respectively.They are placed in contact with the curved surface to wards 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)

A concavo-convex lens has refractive index 1.5 and the radii of curvature of its surfaces are 10 cm and 20 cm. The concave surface is upwords and is filled with oil of refractive index 1.6. The focal length of the combination will be

A concavo-convex lens has refractive index 1.5 and the radii of curvature of its surfaces are 10 cm and 20 cm. The concave surface is upwords and is filled with oil of refractive index 1.6. The focal length of the combination will be