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A converging beam of rays is incident on a diverging lens. Having passed through the lens the rays intersect at a point 15 cm from the lens on the opposite side. If the lens is removed the point where the rays meet will move 5 cm closer to the lens. The focal length of the lens is

  • Option 1)

    - 10 cm

  • Option 2)

    20 cm

  • Option 3)

    - 30 cm

  • Option 4)

    5 cm

 

Answers (2)

best_answer

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.

 u=+10 cm

V=+15 cm \frac{1}{f}=\frac{1}{v}-\frac{1}{\mu}

\frac{1}{f}=\frac{1}{15}-\frac{1}{10}=\frac{-1}{30}

=>f=-30 cm


Option 1)

- 10 cm

This is incorrect option 

Option 2)

20 cm

This is incorrect option

Option 3)

- 30 cm

This is correct option 

Option 4)

5 cm

This is incorrect option 

Posted by

Aadil

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u=+ (15-5) = +10cm ,v=+15cm 

1/f=1/v- 1/u

1/15-1/10=-1/30

f= -  30cm

Posted by

pratibha

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