The power (P) of a lens is defined as the reciprocal of its focal length (f), when the focal length is measured in meters. The unit of power is the dioptre (D), where 1 D = 1 m⁻¹. The formula is P = 1/f. For a concave lens, the focal length is conventionally considered negative. Given the magnitude of the focal length of the concave lens is 2 m, the focal length f = -2 m. Power P = 1 / (-2 m) = -0.5 m⁻¹ = -0.5 dioptre (D).