Question

An object is placed infront a convex lens at such a
distance that its real image of the same size is formed.
When the object is displaced 16 cm towards the
lens, still a real image is formed but now its size is
3 times the size of the object. Calculate the focal
length of the lens.
​

Answer 1

An object placed at a distance of 16cm from a convex lens produces an image of magnification m(m>1). If the object is moved towards the lens by 8cm then again an image of magnification m is obtained. Find the numerical value of focal length of the lens.

ANSWER

Since,

v

1

−

u

1

=

f

1

v

u

−1=

f

1

m=

u

v

=

f+u

f

given, u

1

=−16m

1

=m

u

2

=−8m

2

=−m

f−16

f

=

f−8

−f

f

2

−8f=−f

2

+16f

2f

2

−24f=0

f

2

−12f=0

So

f=12cm

Answer 2

Answer:

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