A biconvex lens has focal length `(2)/(3)` times the radius of curvature of either surface. Calculate refraction index f material of the lens.

Explanation

Here, `R_1 = R, R_2 = -R, f = (2)/(3) R`,
`mu = ?`
`(1)/(f)=(mu - 1)((1)/(R_1) - (1)/(R_2))`
`(3)/(2 R)=(mu - 1)((1)/( R) + (1)/( R)) = (mu - 1) (2)/(R )`
:. `mu - 1 = (3)/(4) = 0.75, mu = 0.75 + 1 = 1.75`.