A concave lens forms the image of an object such that the distance between the o

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A concave lens forms the image of an object such that the distance between the object and image is 10 cm and the magnification proceed is

\frac{1}{4}

. The focal length of the lens will be

Options:

A . 8.6 cm

B . 6.2 cm

C . 10 cm

D . 4.4 cm

Answer: Option D
:
D
Concave lens forms the virtual image of a real object. So let
u = -4x and v = -x then 3x = 10 cm
or

x = \frac{10}{3}

cm

∴

u = - \frac{40}{3} c m

And

v = - \frac{10}{3} c m

Substituting in

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

We get

\frac{1}{f} = \frac{- 3}{10} + \frac{3}{40}

or

f = - \frac{40}{9}

or f = – 4.4 cm

A Concave Lens Forms The Image Of An Object Such That The Di...

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