Question

An eye specialist prescribes a number $4.5D$ to a person for his glasses. What is the nature of the lens? What is the focal length of the lens?

Answer 1

• The power of the doctor's prescribed glasses is positive .therefore it is a convex lens.
• Now the relationship between power and focal length of the lens is

$$\begin{gathered} Poweroflens=\frac{100}{focallengthoflens} \\ \Rightarrow +4.5=\frac{100}{focallengthoflens} \\ \Rightarrow focallengthofthelens=\frac{100}{4.5} \\ \Rightarrow focallengthofthelens=+22.22cm \end{gathered}$$

Hence the doctor has prescribed the person a convex lens with a $+22.22cm$focal length.