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CBSE Class 10 Science 2015 Term II Delhi Set 2

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Question : 11
Total: 11
The image of a candle flame placed at a distance of 30‌cm from a spherical lens is formed on a screen placed on the other side of the lens at a distance of 60‌cm from the optical centre of the lens. Identify the type of lens and calculate its focal length. If the height of the flame is 3‌cm, find the height of its image.
Solution:  
Given:
h1=+3‌cm,u=−30‌cm
v=+60‌cm,f=‌ ?, ‌h′=‌ ? ‌
‌
1
f
=‌
1
v
−‌
1
u
=‌
1
60
−‌
1
−30

‌
1
f
=‌
+3
60

∴‌‌f=+20‌cm
‌
Therefore, the lens is convex.
h′‌=‌
v
u
×h=‌
+60‌cm
−30‌cm
×3‌cm
h′‌=−6‌cm
Here, negative sign indicates that the image formed is inverted. Therefore, height of image of candle flame is 6‌cm.
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