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 A thin convex lens of focal length ‘f’ is put on a plane mirror as shown in the figure. When an object is kept at a distance ‘a’ from the lens - mirror combination, its
image is formed at a distance  \frac{a}{3}  in front of the combination. The value of ‘a’ is :


  • Option 1)


  • Option 2)


  • Option 3)


  • Option 4)



Answers (1)


As we learnt in

Thin lens formula -

\frac{1}{v}-\frac{1}{u}= \frac{1}{f}


- wherein

u \, and \, v are object and image distance from lens.


 When object is kept at distance a from thin convex lens.

By lens formula

\frac{1}{v}=\frac{1}{f}-\frac{1}{a}                                    (i)

Mirror forms image at equal distance from mirror

From lens formula



After solve we get 

a = 2f  

Correct option is 2.



Option 1)


This is an incorrect option.

Option 2)


This is the correct option.

Option 3)


This is an incorrect option.

Option 4)


This is an incorrect option.

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