Question

a real image of 4/5 size of obj is formed 18 cm from a lens . Calculate focal lenghth of lens

Answer 1

Answer:

Given that,

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

Image distance, v = +18 cm

Magnification,

So,

Using lens formula :

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

Explanation:

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Answer 2

Answer:

Given that,

v=18cm

We know that,

f

1

=

v

1

+

u

1

where,

focal length = f

distance of object from lens =u

distance of image from lens = v

Now, the magnification

m=−

u

v

5

4

=

u

18

u=−22.5cm

f

1

=

v

1

+

u

1

f

1

=

18

1

−

22.5

1

f=90cm

Hence, the focal length is 90 cm