The focal length of a concave lens is 0·5 m. The power of the lens is

The power of a lens (P) is defined as the reciprocal of its focal length (f) when measured in meters, expressed by the formula P = 1/f [1]. The SI unit for lens power is the dioptre (D). According to standard sign conventions in optics, the focal length of a convex (converging) lens is positive, while the focal length of a concave (diverging) lens is negative. In this problem, the lens is concave with a focal length of 0.5 m. Applying the sign convention, f = -0.5 m. Substituting this into the power formula gives P = 1 / (-0.5 m), which equals -2.0 D. Therefore, a concave lens with a focal length of 0.5 m has a power of -2.0 D, indicating its diverging nature.

Sources

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