NEET 2015 Physics question
Two identical thin plano-convex glass lenses (refractive index 1.5) each having radius of curvature of 20 cm 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 :-
Answer: (2)
Solution
The system is equivalent to three thin lenses in contact: two plano-convex glass lenses and, between them, a biconcave oil lens (the oil fills the gap between the two convex surfaces). Each glass lens: , and likewise
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