Question

A pin 3 cm long is placed at a distance of 24 cm from a convex lens of focal length 18 cm. The pin is placed perpendicular to the principal axis. Find the size of the image.

• A. 10 cm
• B. 9 cm
• C. 8 cm
• D. 5 cm

Answer 1

The correct option is B

9 cm

Here, u = - 24 cm, f = + 18 cm , n = ?

Using the len's formula, $\frac{1}{v}$ - $\frac{1}{u}$ = $\frac{1}{f}$ we get

$\frac{1}{v}$ - $\frac{1}{-24}$ = $\frac{1}{+18}$

v = 72 cm

The image is formed 72 cm from the lens on the other side. So the image is real.

Magnification, m = $\frac{h^{/}}{h}$ = $\frac{v}{u}$ = $\frac{72}{-24}$ = - 3

$h^{/}$ = - 3 $\times$ h = - 3 $\times$ 3.0 = - 9 cm

The image is 9 cm in size. Negative sign shows that the image is inverted.