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Q.

A convex lens of focal length 20 cm is cut into two equal parts so as to obtain two


plano-convex lenses as shown in the figure. The two parts are then put in contact as shown in


the figure. Which of the following is the focal length of the combination?


Question Image

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a

0

b

5 cm

c

10 cm

d

20 cm 

answer is D.

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Detailed Solution

The focal length of the combination is 20 cm.
Question ImageIf f is the focal length of each plano-convex lens then
1F = 1f + 1f = 2f
f=2× 20=40 cm
Now in fig c two plano-convex lenses are put together,
So focal length of combination
⇒1F=2f
⇒1F = 240
⇒1F = 120
Hence F=20 cm
 
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