Question
11.36.Two identical thin plano-convex 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 intervene in g space is filled with oil of refractive index 1.7. The focal length of the combination is A. -20 cm B. -25 cm C. -50 cm D. 50 cm