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A convex lens, of focal length 30 cm, a concave lens of focal length 120 cm, and a plane mirror are arranged as shown.  For an object kept at a distance of 60 cm from the convex lens, the final image, formed by the combination, is a real image, at a distance of :

  • Option 1)

    60 cm from the convex lens

  • Option 2)

    60 cm from the concave lens

  • Option 3)

    70 cm from the convex lens

  • Option 4)

    70 cm from the concave lens

 

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.

 

 

Len's formula is given by

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

For convex lens = \frac{1}{30}=\frac{1}{v}+\frac{1}{60}=\frac{1}{60}=\frac{1}{v}

For concanve lens = \frac{1}{-120}=\frac{1}{v}-\frac{1}{40}=\frac{1}{v}=\frac{1}{60}

Virtual object 10 cm behind plane mirror.

Hence real image 10 cm infront of mirror or 60 cm from convex lens. 

Correct option is 1.

 


Option 1)

60 cm from the convex lens

This is the correct option.

Option 2)

60 cm from the concave lens

This is an incorrect option.

Option 3)

70 cm from the convex lens

This is an incorrect option.

Option 4)

70 cm from the concave lens

This is an incorrect option.

Posted by

Vakul

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