A person needs a lens of power $-4.5\ D$ for 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?

$(a)$. She is suffering from myopia because the power of the lens is negative. In other words, it can be said nearsightedness, when a person can see objects clearly near to him/her but the objects placed far away seems to be blurry.

$(b)$. We know the relationship between the power of the lens and its focal length.

$\boxed{P=\frac{1}{f}}$

Here, $P\rightarrow$ power of the lens

$f\rightarrow$ focal length of the lens

Here given, power of the lens $P=-4.5\ D$

Therefore, focal length of the lens, $f=\frac{1}{P}$

$=\frac{1}{-4.5}\ m$

$=-0.22\ m$

$=-22\ cm$            [as $1\ m=100\ cm$]

$(c)$. As the focal length of the lens is negative, so the lens is a concave lens.