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CBSE Class 10 Science 2015 Term II Outside Delhi Set 1

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Question : 23
Total: 36
One half of a convex lens of focal length 10 cm is covered with a black paper. Can such a lens produce an image of a complete object placed at a distance of 30‌cm from the lens? Draw a ray diagram to justify your answer. A 4‌cm tall object is placed perpendicular to the principal axis of a convex lens of focal length 20‌cm. The distance of the object from the lens is 15‌cm. Find the nature, position and size of the image.
Solution:  
Yes, it can produce an image of a complete object placed at a distance of 30‌cm from the lens.
Given : h1=4‌cm,f=+20‌cm,u=−15‌cm
We know that,
‌
1
f
=‌
1
v
−‌
1
u

‌
1
v
=‌
1
f
+‌
1
u
=‌
1
20
‌‌‌
1
15
=‌
34
60
=‌
1
60

∴ Image distance, v=−60‌cm.
Image will be virtual, erect and placed in the direction of the object.
Now, Magnification, m=‌
h2
h1
=‌
v
u

h2=‌
v
u
×h1=‌
−60
−15
×4=+16‌cm.
The size of the image is 16‌cm.
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