The power of a convex lens of focal length 20 cm in dioptre unit is

The correct answer is Option (3) → 5

The power of a lens is the reciprocal of its focal length expressed in metres; the unit is the dioptre (D).

    \( f = 20\,\mathrm{cm} = 0.20\,\mathrm{m} \)

    \( \displaystyle P = \frac{1}{f} = \frac{1}{0.20\,\mathrm{m}} = 5\,\mathrm{D} \)