Question

A real image, 4/5 size of the object is formed 18 cm from a lens. calculate the focal length of the lens.

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Answer 1

A real image 4/5 size of object is formed at 18 cm from a lens.

Image distance, v = +18 cm

Magnification,

$m = \frac{h1}{h} = \frac{4}{5}$

So,

$\frac{h1}{h} = \frac{ - 4}{5} = \frac{v}{u}$

$4u = - 5v$

$4u = - 5 \times 18$

$u = - 22.5cm$

Using lens formula :

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

$\frac{1}{f} = \frac{1}{(18)} - \frac{1}{( - 22.5)}$

$f = 10$

So, the focal length of the lens is 10 cm.