The power of lens is -2 D. What is its focal length ?

The power of a lens (P) is defined as the reciprocal of its focal length (f) expressed in meters [1][t1]. The mathematical relationship is given by the formula P = 1/f or f = 1/P [t2][t7]. Given the power of the lens is -2 D, the focal length is calculated as f = 1 / (-2) = -0.5 m [t7]. The negative sign indicates that the lens is a diverging or concave lens [t4][t10]. While the magnitude of the focal length is 0.5 m, it can also be expressed as 50 cm [t7]. Among the provided options, 0.5 m (Option 4) correctly represents the magnitude of the focal length derived from the standard diopter formula where the unit of length is meters [t5][t9].

Sources

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