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CBSE Class 10 Science 2016 Term II Outside Delhi Set 1

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Question : 23
Total: 36
(a) Define optical centre of a spherical lens.
(b) A divergent lens has a focal length of 20 cm. At what distance should an object of height 4‌cm from the optical centre of the lens be placed so that its image is formed 10‌cm away from the lens. Find the size of the image also.
(c) Draw a ray diagram to show the formation of image in above situation.
Solution:  
(a) Optical centre is a point on a lens through which ray of light passes undeviated, without suffering any refraction.
(b) Given: f=−20‌cm,h0=4‌cm,v=−10‌cm We know that,
‌
1
f
‌=‌
1
v
−‌
1
u

−‌
1
20
‌=−‌
1
10
−‌
1
u

‌
1
u
‌=−‌
1
10
+‌
1
20

‌
1
u
‌=‌
−2+1
20
=‌
−1
20

u‌=−20‌cm
Now,
m‌=‌
hi
ho
=‌
v
u

‌
hi
4
‌=‌
−10
−20
⇒‌‌ 2 h_i\;=4 $
⇒hi=2‌cm
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