Question

A 3 cm high object is placed at a distance of 80 cm from a concave lens of focal length 20 cm. Find the size of the image formed.

• A. 0.1 cm
• B. 0.4 cm
• C. 0.2 cm
• D. 0.6 cm

Answer 1

The correct option is D 0.6 cm

Given:
Object distance, $u=-80cm$
Focal length, $f=-20cm$
Object height, $h=+3cm$

Using the lens formula,
$\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$
$\frac{1}{v}-\frac{1}{-80}=\frac{1}{-20}$
$v=-16cm$

Using formula for magnification,
$m=\frac{h^{\prime }}{h}=\frac{v}{u}$
$\frac{h^{\prime }}{+3}=\frac{-16}{-80}$
$h^{\prime }=\frac{h}{5}=\frac{3.0}{5}=0.6cm$

Height of the image is 0.6 cm. Positive sign shows that the image is erect.