The power of a biconvex lens is 10 dioptre and the radius of curvature of each surface is 10 cm. Then the refractive index of the material of the lens is :
(1) $3\over2$
(2) $4\over3$
(3) $9\over8$
(4) $5\over3$
(1) $3\over2$
(2) $4\over3$
(3) $9\over8$
(4) $5\over3$