Diameter of a Plano-convex lens is 6 cm and the thickness at the center is 3 mm. If the speed of light in the material of the lens is 2 x 10^8 m/sec, what is the focal length of the lens?

diameter = 2 Radius (R)

 2R = 6

 R = 3cm

1/f = (u-1)/R      ......1      

this is the relation bw focal length of plano convex lens & u(refractive index) ...

u = c/v = 3*10⁸/2*10⁸ = 1.5

now put u in eq 1 , will get focal length