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CBSE Class 10 Science 2016 Term II Outside Delhi Set 2

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Question : 12
Total: 12
(a) Define focal length of a spherical lens.
(b) A divergent lens has a focal length of 30‌cm. At what distance should an object of height 5‌cm from the optical centre of the lens be placed so that its image is formed 15‌cm away from the lens? Find the size of the image also.
(c) Draw a ray diagram to show the formation of image in the above situation.
Solution:  
(a) The distance between the optical centre and focus of a spherical lens is called focal length.
(b) Given : f=−30‌cm,hi=5‌cm,hi= ?,
v=−15‌cm,u=‌ ? ‌
We know that,
or
‌
1
f
‌=‌
1
v
−‌
1
u

‌
1
u
‌=‌
1
v
−‌
1
f

u‌=‌
vf
f−v

‌=‌
−15×−30
−30+15
=−30‌cm.
⇒‌‌u=‌
vf
f−v

Now,
‌ Now, ‌‌‌‌m=‌
v
u
=‌
hi
h0

⇒hi=‌
v
u
×h0=‌
−15
−30
×5=2.5‌cm.
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