An optician prescribes a power = -0.5 dioptre. The corresponding lens must be a

The power of a lens (P) is defined as the reciprocal of its focal length (f) in meters, expressed by the formula P = 1/f [1]. In this case, the prescribed power is -0.5 dioptre. Using the formula f = 1/P, we calculate f = 1/(-0.5) = -2 meters. By optical convention, a negative power and negative focal length signify a diverging lens, which is a concave lens [1]. Conversely, convex lenses are converging and possess positive power and focal length values. Therefore, a power of -0.5 dioptre corresponds to a concave lens with a focal length of 2 meters. The negative sign specifically denotes the diverging nature of the lens, while the magnitude of 2 meters represents the distance from the optical center to the virtual focal point [2].

Sources

1. [1] Science , class X (NCERT 2025 ed.) > Chapter 9: Light – Reflection and Refraction > 9.3.8 Power of a Lens > p. 157
2. [2] Science , class X (NCERT 2025 ed.) > Chapter 9: Light – Reflection and Refraction > What you have learnt > p. 159