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CBSE Class 10 Science 2018 Solved Paper

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Question : 26
Total: 27
An object of height 4.0‌cm is placed at a distance of 30‌cm from optical centre ' O ' of a convex lens of focal length 20‌cm. Draw a ray diagram to find the position and size of the image formed. Mark optical centre ' O ' and principal focus ' F ' on the diagram. Also find the approximate ratio of size of image to the size of object.
Solution:  
Given, f=+20‌cm,u=−30‌cm h0=4‌cm
We know that,
‌
1
f
=‌
1
v
−‌
1
u

‌
1
20
=‌
1
v
−‌
1
−30

‌
1
v
=‌
1
60

⇒‌‌v=60‌cm
‌
hi
h0
=‌
v
u

‌
hi
4
=‌
60
−30

So,⇒‌‌hi=−8‌cm
Thus, the height or size of the image is 8‌cm. The minus sign shows that this height is in the downward direction, that is, the image is formed below the axis.
Ratio of size of image to object =−‌
8
4
=−2
So image is enlarged beyond 2F2.
Object between F1 and 2F1.
Image is formed beyond 2F2, real, inverted.
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