# Aaconverging lens of focal length 15 mm is used as a magnifying glass for inspecting stamp. A rectangular stamp which is 20mm 40mm is placed 30mm from the lens find the length of the image of the 20mm side of the stamp?

Jul 17, 2018

Lens formula is

$\frac{1}{v} - \frac{1}{u} = \frac{1}{f}$

Where

$\text{object distance} \left(u\right) = = - 30 m m$

$\text{focal length} \left(f\right) = + 15$

"image distance" (v)==?

Now inserting in the eqution we get

$\frac{1}{v} - \frac{1}{- 30} = \frac{1}{15}$

$\implies \frac{1}{v} = \frac{1}{15} - \frac{1}{30} = \frac{1}{30}$

$\implies v = 30 m m$

Hence magnification $\left(m\right) = \frac{v}{u} = \frac{30}{30} = 1$

So the image is real and the length of 20mm side of the stamp in the image will be same, 20mm.