Question

An object of height 6 cm is placed perpendicular to the principal axis of a concave lens of focal length 5 cm. Use lens formula to determine the position, size and nature of the image if the distance of the object from the lens is 10 cm.

Answer 1

Given : $u=-10$ cm $f=-5$ cm $h_o=6$ cm

Using lens formula : $\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$

$\therefore$ $\frac{1}{v}-\frac{1}{(-10)}=\frac{1}{-5}$

$⟹v=-\frac{10}{3}=-3.33$ cm (- sign shows that image is virtual)

Also $m=\frac{h_i}{h_o}=\frac{v}{u}$

$\therefore$ $\frac{h_i}{6}=\frac{\frac{-10}{3}}{-10}$

$⟹h_i=+2$ cm (+ sign shows that image is erect)