CBSE Class 10 Science 2017 · Paper 31/2/1 · Question 16

Light - Reflection and Refraction 3 marks Medium

The magnification of an image formed by a lens is

- 1

. If the distance of the image from the optical centre of the lens is 25 cm, where is the object placed ? Find the nature and focal length of the lens. If the object is displaced 15 cm towards the optical centre of the lens, where would the image be formed ? Draw a ray diagram to justify your answer.

Answer

Object at $u = - 25$ cm ( $2 F_{1}$ ); convex lens, $f = + 12.5$ cm; after shifting, $v^{'} = - 50$ cm, a virtual, erect image five times as tall.

The object is placed 25 cm from the optical centre on the other side of the lens from the image (

u = - 25

cm), that is at

2 F_{1}

, and the image is at

2 F_{2}

. The lens is converging (convex), with focal length

f = + 12.5

cm. When the object is moved 15 cm towards the optical centre it is 10 cm from the lens (

u = - 10

cm), inside the focal length. The image then forms 50 cm from the lens on the same side as the object (

v = - 50

cm); it is virtual, erect and five times as tall (

m = + 5

). In the ray diagram the two emergent rays diverge and their backward extensions meet at that point.

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CBSE Class 10 Science 2017 · Paper 31/2/1 · Question 16

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