The Power of a convex lens of focal length 25 cm is :

The correct answer is Option (3) → \( +4\,\mathrm{D} \)

The power of a lens is the reciprocal of its focal length in metres, \( P = 1/f \), measured in dioptres. A convex (converging) lens has a positive focal length, so its power is positive.

    \( \displaystyle f = +25\,\mathrm{cm} = +0.25\,\mathrm{m} \)

    \( \displaystyle P = \frac{1}{f} = \frac{1}{0.25\,\mathrm{m}} = +4\,\mathrm{D} \)