Question

An extended object of size $2\text{mm}$ is placed on the principal axis of a converging lens of focal length $10\text{cm}$ . It is found that when the object is placed perpendicular to the principal axis the image formed is $4\text{mm}$ in size. The size of image when it is placed along the principal axis is ( in $\text{mm})$.

Answer 1

When the object is placed perpendicular to the optical axis:

From the definition of Lateral magnification we know that,

Image size$\left(I\right)$ = $m\times$ object size$\left(O\right)$

$\therefore m=\frac{4}{2}=2$

Now, when the object is placed along the optical axis:

From the definition of longitudinal magnification we can say that,

Image size $\left(I^{\prime }\right)$ $=m^2\times$ object size$\left(O\right)$
$I^{\prime }=(2{)}^2\times 2=8\text{mm}$

Accepted answer : $8,8.0$