$A$ person needs a lens of power $-4.5\, D$ for the correction of her vision.
$(a)$ What kind of defect in vision is she suffering from?
$(b)$ What is the focal length of the corrective lens?
$(c)$ What is the nature of the corrective lens?

(N/A) The person is suffering from $Myopia$ (near-sightedness) because the power of the lens is negative.
$(b)$ The focal length $f$ is given by the formula $f = \frac{1}{P}$,where $P$ is the power in diopters $(D)$.
$f = \frac{1}{-4.5\, D} = -0.222\, m$ or $-22.2\, cm$.
$(c)$ Since the power and focal length are negative,the nature of the corrective lens is a $Concave$ lens.