Diameter of a plano-convex lens is 6 cm and thickness at the center is 3 mm. If the speed of light in material of lens is 2×108ms−1 . The focal length of the lens is in ……..cm.

Refractive Index,  n=32[∵n=cv]   Apply pythagoras theorem,  32+(R−0.3)2=R2   (all dimensions are in cm) ⇒R=15 cm  Apply Lens Maker formula,   ∴1f=(32−1)(115)∵1f=(n−1)(1R1−1R2) and R1=∞  R2=−15  ⇒f=30 cm