Diameter of a plano-convex lens is 6 cm and thickness

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 × 108 m/s, the focal length of the lens is:

  1. 10 cm
  2. 15 cm
  3. 20 cm
  4. 30 cm

Answer

n = 3/2

32 + (R - 3)2 = R2

32 + R2 + 2R(3) - (3)2 = R2

R ≈ 15 cm

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

f = 30 cm

The correct option is D.