If the focal length of the biconvex lens is 25 cm, then the power of the lens will be

The power of a lens (P) is defined as the reciprocal of its focal length (f) when measured in meters. The formula is expressed as P = 1/f (in meters). In this case, the focal length is given as 25 cm. Converting this to meters yields 0.25 m (25/100) [1]. Applying the formula, P = 1 / 0.25, which equals 4 dioptres [1]. Furthermore, a biconvex lens is a converging lens. According to optical sign conventions, converging (convex) lenses have a positive focal length and thus a positive power, whereas diverging (concave) lenses have negative power. Therefore, the power of a biconvex lens with a 25 cm focal length is +4 dioptre.

Sources

1. [1] Science , class X (NCERT 2025 ed.) > Chapter 9: Light – Reflection and Refraction > 9.3.8 Power of a Lens > p. 157