Question

An object and a screen are fixed 50 cm apart. When a converging lens is placed at two positions A and B between the object and screen, the image of object is found on screen. If the distance between A and B is 10 cm, then focal length of lens is


(1) 12 cm


(3) 8 cm


(2) 10 cm


(4) 6 cm

Answer 1

12th

Physics

Ray Optics and Optical Instruments

Image Formation by Lenses

An object is placed 50 cm f...

PHYSICS

An object is placed 50 cm from a screen as shown in fig. A converging lens is moved such that line MN is its principal axis. Sharp images are formed on the screen in two positions of lens separated by 30 cm. Find the focal length of the lens in cm

question

November 22, 2019avatar

Yashi Tirdiya

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ANSWER

Let an image be formed at the screen when the lens is at a distance of x from object. Then applying Lens equation gives,

50−x

1

−

−x

1

=

f

1

x

2

−50x+50f=0

Solution for this quadratic equation is,

x

1

=

2

50+D

and x

2

=

2

50−D

As given, x

1

−x

2

=50

Hence D=

50

2

−200f

=30

f=8cm