Diameter of a plano – convex lens is 6 cm and thickness at the centre is 3 mm. If speed of light in material of lens is 
m/s, the focal length of the lens is:
(1) 15 cm (2) 20 cm (3) 30 cm (4) 10 cm
[JEE Main 2013]
Solution
t = 3 mm = 0.3 cm
Now, 
Hence, (3).