Question

State the nature of the lens and the focal length if its power is $+4D$.

Answer 1

• The power of the lens is positive which means it is representing a convex lens.
• Now there is a relationship between the power of the lens and the focal length.

$Poweroflens=\frac{100}{focallengthoflens}$

Now we can write $$\begin{gathered} +4=\frac{100}{focallengthoflens} \\ \Rightarrow focallengthoflens=\frac{100}{4} \\ \Rightarrow focallengthoflens=25cm \end{gathered}$$

Hence the focal length of the lens is $25cm$.