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 2 × 10^{8} m/s, the focal length of the lens is:

- 10 cm
- 15 cm
- 20 cm
- 30 cm

**Answer**

n = 3/2

3^{2} + (R - 3)^{2} = R^{2}

3^{2} + R^{2} + 2R(3) - (3)^{2} = R^{2}

R ≈ 15 cm

1/f = ((3/2)-1)(1/15)

f = 30 cm

**The correct option is D.**