(a)

Write the expression for the magnification produced by a lens in terms of object distance and image distance.

(b)

A $\displaystyle 4$ cm tall object is placed perpendicular to the principal axis of a convex lens of focal length $\displaystyle 20$ cm. Calculate the size of the image formed, if the distance of the object from the lens is $\displaystyle 10$ cm.

(a) Magnification, \(\displaystyle \mathrm{m}=\frac{\text { Image distance }}{\text { Object distance }} / \mathrm{m}=\frac{\mathrm{v}}{\mathrm{u}}\)

(b) \[\begin{aligned} & \mathrm{h}_{\mathrm{o}}=+4 \mathrm{~cm} \\ & \mathrm{f}=+20 \mathrm{~cm} \\ & \mathrm{u}=-10 \mathrm{~cm} \\ & \mathrm{~h}_{\mathrm{i}}=? \\ & \frac{1}{\mathrm{v}}-\frac{1}{\mathrm{u}}=\frac{1}{\mathrm{f}} \\ & \frac{1}{\mathrm{v}}=\frac{1}{\mathrm{f}}+\frac{1}{\mathrm{u}} \\ & \frac{1}{\mathrm{v}}=\frac{1}{20}+\frac{1}{-10} \\ & \frac{1}{\mathrm{v}}=\frac{-1}{20} \\ & \mathrm{v}=-20 \mathrm{~cm} \end{aligned} \]