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Q.

An object is kept at a distance of 4cm from the first focus of a convex lens. A real image is formed at a distance of 9cm from its second focus. Then focal length of that lens is

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a

6.5 cm

b

13 cm

c

6 cm

d

36 cm

answer is C.

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Detailed Solution

Question Image

 

 

 

 

Given that

OF1=4cm IF2=9cm

u=−(4+f) v=(f+9)

We can have lens formula

1v−1u=1f

⇒19+f+14+f=1f

⇒(4+f+9+f)(9+f)(4+f)=1f

⇒(13+2f)f=36+13f+f2

⇒13f+2f2=36+13f+f2

⇒f2=36∴f=6cm

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An object is kept at a distance of 4cm from the first focus of a convex lens. A real image is formed at a distance of 9cm from its second focus. Then focal length of that lens is