A doctor has prescribed a corrective lens of power +1.5 D. Find the focal length of the lens. Is the prescribed lens diverging or converging?

a) Given,

    Power of lens, $\mathrm{P} = +1.5 \mathrm{D}$

    Focal length of a lens, $\mathrm{f}=?$

    Equation of power of lens is,

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

    Substituting the values,

                                                 $\Rightarrow \mathrm{f}=\frac{1}{\mathrm{P}}=\frac{1}{1.5}$

    On simplification,

                                                  $\Rightarrow \mathrm{f}=0.66 \mathrm{m}$

   Positive f indicates a convex lens. 

b) The power of converging lens is positive and that of a diverging lens is negative.

Since the focal length of lens is $\mathrm{f}=0.66 \mathrm{m}$, the lens prescribed is a converging lens.